What are your most commonly used/ingenious/favorite house rules used by your group, now or in the past? Would love some commentary on our rules while you're at it.

Here's ours:

1. These simply speed up the minutia for gameplay sake:
   • Visibility/Darkness: Light via camp fires, torches, and lanterns are assumed (unless otherwise implicitly indicated by the DM). Just make sure your character possesses the required implements of light.
   • Rations, dining out, or hunting and gathering will be assumed, as long as: each character starts a cavern/dungeon, or the like, off with 3 rations in their inventory, each character maintains 5 gold each while in a town or city, a member of the party has a hunting/gathering skill and/or at least one party member makes a Survival check.
   • Only verbal and somatic components of spell casting will be recognized for this campaign (unless implicitly stated otherwise by the DM).
2. Drinking a potion consists of a Bonus Action, whereas administering a potion requires an Action.
3. Prone: If you stand up and attack in the same turn, take a -2 modifier to your attack roll due to recovery effort.
4. Flanking: Attackers (A & B) gain advantage on attack rolls when two or more creatures are melee attacking one defender (D) from opposing sides. Any subsequent attackers (C, E,,, R) will also gain advantage since the defender (D) is already in a “flanked” status. Flanking advantage now includes ranged attacks (R) as long as the ranged attack is not required to propagate the flanking scenario.
5. Moving Around Other Creatures: 
   • You can move through a non-hostile creature’s space.
   • You can move through a hostile creature’s space only if the creature is at least two sizes larger or smaller than you.
   • No more than two creatures can occupy a space simultaneously. 
   • You can only attack from an occupied space if the other creature in the space is currently prone.
   • You cannot end your turn within a shared space with another creature.
     

Here is my House Rules document.  I'll paste a few of them below:

Background Story

Please do not write one! It’s boring. I don’t want to read your back story and usually they restrict group storytelling. During every game session we should be learning a little bit more about your Background and Personality Traits. When you enjoy a movie, tv series, novel, or video game, we learn about each character's background as the story unfolds. It should be the same in a table top game. If you want to have secrets and ideas, that’s fine, but until they appear in a game session they are not canon. The story we tell as a group is your backstory. That lame fanfic you wrote two years ago and keep pointing to when you say “My character wouldn’t do that”, is not your backstory.

Monologue

Once per game session, when you roll a natural 20, you can take a moment to monologue. The game stops for a moment while you create a story using your Background or one of your Personality Traits (Ideal, Bond, Flaw) as the focus. The monologue ends when you tell the group how that now inspired you in this moment of the story. You earn Inspiration.

Long Rest

Long rests take at least 8 hours. Whenever the party takes a long rest, I offer the opportunity for one player to develop their Background Story. This might take place at
night in the the tavern, huddled around a campfire, barricaded in a dungeon room, or any other time during a Long Rest. Only one player per long rest is allowed to develop their Background Story. That player should spend 3-5 minutes roleplaying as his or her character, recounting a story from their past. After the player recounts the Background
Story, every other player (including the DM) may ask you one question to clarify a detail or expand on a topic in your story. When everyone has had a turn, the player
who told their Background Story is awarded “Inspiration”. Players should consult their Background and Personality traits but they can also
randomly roll or chose one of the following topics for a Background Story:

Background Story(d12):

1. Speak about where you came from, how you became an adventurer, and how your Background has influenced your life.
2. Retell the story of your greatest love.
3. Confess a fear that you have been too embarrassed to share until now.
4. Speak about a tragic or heroic event from your youth.
5. Entertain the others with a tale of one of your epic failures or heroic successes.
6. Recount the tale of losing a family member.
7. Surprise your friends by speaking about your greatest desire or goal.
8. Frighten others with a horrific event that happened to you or someone you knew.
9. Something amazing, fantastic, tragic, or horrific happened to you when you were a child. What was it?
10. At least once in your life something has challenged your Ideal. What was one the most difficult times this happened?
11. Everyone has formed at least one Bond in the setting. Tell us how the bond was formed, what you would do if you lost this bond, or a time when you almost lost this bond.
12. Name a time when a vice, compulsion, fear, or weakness from your Flaw seriously impacted your life.

Cheers!

Haven't actually used this one yet,but: advantage on ranged attack rolls with a crossbow or rifle while prone.

Quote from DnDNewper >>

Haven't actually used this one yet,but: advantage on ranged attack rolls with a crossbow or rifle while prone.

This would be overpowered; being prone gives all enemies disadvantage to hit you at range. In reality firing from prone like this actually makes it harder to aim as it restricts your ability to turn quickly, you only want to do it when you know roughly where a target is going to be (e.g- you're at the end of a street or gulley expecting enemies to approach from the opposite end), I think it's best negotiated with your DM, but I wouldn't recommend advantage, I'd just cancel the normal disadvantage, this way you still get the benefit of being prone (harder to see/hit from a distance) but without the drawback, but only in a specific circumstance (ambush).

My favourite house rule is critical failures on skill checks; I'm not personally a fan of skills reaching a point where they basically just succeed automatically, I like to feel as though there's always a chance of screwing something up. Even the most experienced, smooth talking bard can accidentally call someone by the wrong name, or say something that causes extreme offence without meaning to. Even the most experienced climber can put a foot wrong, etc. I'm less of a fan of critical successes, but some groups still use those too, to represent someone knowing just the right piece of information by chance, or getting lucky on a jump, negotiation etc.

That rule was specifically made for if a PC/npc wanted to snipe something from a distance,like a IRL sniper. Obviously you shouldn't do this while in the middle of a bar brawls.

I like the idea of this however the rebuttal makes sense too. I think I would add a stipulation that advantage would only apply at a minimum distance (25'?) and within 15 degrees of facing direction. Is this overly complicated? Yes, but it gives the player character the option to do something interesting but confines the option from being over used. Working these things out with the DM is sometimes the most satisfying part of the game.

Quote from blazingwebs >>

I like the idea of this however the rebuttal makes sense too. I think I would add a stipulation that advantage would only apply at a minimum distance (25'?) and within 15 degrees of facing direction. Is this overly complicated? Yes, but it gives the player character the option to do something interesting but confines the option from being over used. Working these things out with the DM is sometimes the most satisfying part of the game.

That's an interesting tweak! One funny thing though about shooting at range while prone is that you already have disadvantage at long range, so going prone in that case actually has no additional cost, so the characters that would really want to cancel the added disadvantage will be ones with the Sharpshooter feat (who ignore the long-range penalty).

In that case I'd maybe suggest any houserule be added to that feat in particular, and be something along the lines of:

• If you are prone at the start of your turn, you may enter a sniping posture. Until the end of the turn you may not move or change facing, and being prone will not impose disadvantage on crossbow and firearm attacks against targets that are at least 80 feet away.

That's about as clean as I think I can get it; 80 feet is quite a substantial distance, but shouldn't be a problem if you're setting up a sniping position, and you have to start prone so it's not something you can easily drop into on a whim.

Good idea! in fact, lt just inspired me to make this: (https://www.dndbeyond.com/feats/402297-trained-sniper)

A suggestion: implement a houserule allowing creatures to voluntary drop prone using their reaction after being targeted by a ranged attack.  The triggering attack resolves normally; after that a creature intelligent enough to do so can Hit The Deck to impose disadvantage on follow-up shots.  Both the PCs and intelligent enemies can do so, partially mitigating the threat of arrow flurries - but also leaving themselves prone, with all the headaches that can bring in combat.

As for favorite houserules?  Hn.  We don't tend to use a lot of formalized bullet-point Quick Rules, stuff simply gets adjudicated at the table.  But there are a few.  In no particular order.

1.) Dual Wield Exception
Two-Weapon Fighting can be done with one 'light' weapon, as opposed to two, and drawing two weapons instead of one at the start of combat is an assumed free action.  Dual Wielder has been adjusted to compensate.  This allows common and thematic assymetric dual-wield options such as rapier/dagger, longsword/shortsword, or similar.  The DM can step on obnoxious abuse of this rule, but for the most part Two-Weapon Fighting is so bad that boosting it for style purposes has yet to break the game.

2.) Enhanced Spell Foci (Gandalfing/Wandslinging)
Wielding a spell focus allows a character to fulfill somatic spell components even if the focus is not replacing material components for the spell.  This remains true even if the character is wielding a weapon in their other hand, because sword/staff or sword/wand battlemages are just freaking awesome.  Spell foci can take whatever form the character desires, though wielding one properly is assumed to require the full use of the hand - no using a crystal ring as a spell focus to keep the hand 'open'.  Spell foci are also able to be located as magic items, though this is much more rare than typical weapons.

3.) Starter Freat
All characters are permitted to select a feat at level 1.  This feat cannot be a combat-related feat, or any of the other 'Power' feats.  Ideally this feat is a species-based feat, or a noncombat  'silly' feat which enhances the character's history or aptitudes such as Linguist, Actor, or Athlete.  Variant Human can suck eggs.

4.) Shield Master+
Any character which selects the Shield Master feat can use the shield as a weapon.  Shields deal 1d4+STR bludgeoning damage; the character is considered proficient with this attack.  Nobody qualified to call themselves a 'Shield Master' treats shields as a non-proficient improvised weapon.

5.) Enhanced Standard Progression (this is a new method we're prototyping for our upcoming Exandria campaign.  We haven't field-tested this one yet, but everybody's pretty excited for it.)

Characters begin the game with either Standard Array stats or by-the-book Point Buy stats (DM decision).  As the character progresses, any time they attain a 'standard' ASI granted by their class, they must take it as an ASI and boost their stats - they cannot substitute a feat for the ASI.  Instead, at character levels 4, 8, 12, 16, and 19 (also 1, due to Starter Freat),  the character gains one feat of their choice.  Feats are organized into a loose tier list; tier 2 feats cannot be taken until level 8, and tier 3 cannot be taken until level 12.  The 'bonus' ASIs granted by the rogue and fighter classes are exempt and can be taken as either an ASI or a feat, as per the normal rules.

I’m actually trying out sort of the opposite to your 5), Yurei. I’ve been telling them that when their class gives them a “standard” ASI, they have to take a feat (exempting the fighter/rogue “bonus” ASIs as you do). I’m giving out ”experience points” at the end of every session for playing into ideals, bonds, and flaws (stealing from games like Dungeon World and Blades in the Dark for this) that they’ll use to purchase ability score increases when they level up.

Here are 5 of my house rules:

1) Prone Punishment. Being prone is currently too cheap to get rid of. This is a simple fix to make it a bit more punishing. 

• If a creature stands up from being prone, each creature within 5 feet of you can make an opportunity attack. 

2) Subclass Swap. Sometimes a player wants to play a character that there currently isn't an option to play in the game. When homebrewing isn't an option, I will occasionally allow this if a better subclass/class for their character is released. Here are some examples of my using this in game:

• A Druid in my Spelljammer campaign wanted to be bonded to Wildspace, but there were currently no options for this when the campaign started. I homebrewed a "Space" terrain for Circle of the Land Druids that included some "space" themed spells like Hunger of Hadar, Armor of Agathys, and other spells. Then, the Circle of Stars Druid came out in an Unearthed Arcana, and the character was allowed to change to this subclass after being caught in a time vortex. 
• A Gnome Wizard of the School of Transmutation wanted to be a crafting character with a mechanical servant. When ERftLW came out, I allowed them to undergo an in-game ritual to transform them into a Battle Smith Artificer. 

3) Bloodied and Bruised. Badly damaged monsters become noticeably hurt.

• This is borrowed from previous editions. When a creature reaches half their hit point maximum or lower from damage, they become bloodied. This is only a visual cue to show players the state of the creature's hit points. 

4) Free Feat. Simple feature to boost characters at early levels.

• At level one, each character gets a free feat, but it must be one that doesn't give a bonus to an ability score. The character must have a backstory-based reason for having this feat.

5) Fixing Sorcerers. Purely a few homebrew rules to fix the sorcerer class. 

• Subclass Spells. Each subclass automatically gets a list of known spells, two per level up to level 5 spells. 
• Sorcerous Recovery. Once a day, during a short rest, a sorcerer may regain sorcery points equal to half their sorcerer level. 
• Additional Metamagic. The amount of metamagic a sorcerer knows scales with their Proficiency Bonus. 
• Homebrew Metamagic. I have some homebrew metamagic options, such as being able to cast two leveled spells on a turn if one is a bonus action and you expend sorcery points, giving advantage on concentration checks for spells that require concentration, and expending hit dice and sorcery points to increase damage. 

There are some of my homebrew rules. 

Quote from Third_Sundering >>

Here are 5 of my house rules:

1) Prone Punishment. Being prone is currently too cheap to get rid of. This is a simple fix to make it a bit more punishing. 

• If a creature stands up from being prone, each creature within 5 feet of you can make an opportunity attack. 

2) Subclass Swap. Sometimes a player wants to play a character that there currently isn't an option to play in the game. When homebrewing isn't an option, I will occasionally allow this if a better subclass/class for their character is released. Here are some examples of my using this in game:

• A Druid in my Spelljammer campaign wanted to be bonded to Wildspace, but there were currently no options for this when the campaign started. I homebrewed a "Space" terrain for Circle of the Land Druids that included some "space" themed spells like Hunger of Hadar, Armor of Agathys, and other spells. Then, the Circle of Stars Druid came out in an Unearthed Arcana, and the character was allowed to change to this subclass after being caught in a time vortex. 
• A Gnome Wizard of the School of Transmutation wanted to be a crafting character with a mechanical servant. When ERftLW came out, I allowed them to undergo an in-game ritual to transform them into a Battle Smith Artificer. 

3) Bloodied and Bruised. Badly damaged monsters become noticeably hurt.

• This is borrowed from previous editions. When a creature reaches half their hit point maximum or lower from damage, they become bloodied. This is only a visual cue to show players the state of the creature's hit points. 

4) Free Feat. Simple feature to boost characters at early levels.

• At level one, each character gets a free feat, but it must be one that doesn't give a bonus to an ability score. The character must have a backstory-based reason for having this feat.

5) Fixing Sorcerers. Purely a few homebrew rules to fix the sorcerer class. 

• Subclass Spells. Each subclass automatically gets a list of known spells, two per level up to level 5 spells. 
• Sorcerous Recovery. Once a day, during a short rest, a sorcerer may regain sorcery points equal to half their sorcerer level. 
• Additional Metamagic. The amount of metamagic a sorcerer knows scales with their Proficiency Bonus. 
• Homebrew Metamagic. I have some homebrew metamagic options, such as being able to cast two leveled spells on a turn if one is a bonus action and you expend sorcery points, giving advantage on concentration checks for spells that require concentration, and expending hit dice and sorcery points to increase damage. 

There are some of my homebrew rules. 

 I really dig 1, 3, & 4. I'd be open to 2 if the need arose.

"Being prone is currently too cheap to get rid of"

Amen to that! Standing up from prone, especially while surrounded by enemies, would be really difficult.

I think if I was trying to write the rule, if there is at least one enemy in melee range, I would make it an athletics or acrobatics check (something like DC10 plus 5 per extra enemy within melee range, or maybe contested against one enemy): Pass, and you stand using half your movement as normal. Fail, and you choose:

• stand, but use all your movement and trigger OA from any enemy in melee range
• stand, but use your action, or
• remain prone

I really like standing from prone costing a fixed movement value...15ft instead of half movement. A monk or barbarian is unfairly affected by the half movement rule.