I have some uses for the find familiar spell I think are pretty OP:

1. If you are blinded/deafened and need to see/hear what is going on you can use an action to use your familiars senses until the start of your next turn. this does take an action however and is not great if you are in combat.

2. you can command the familiar to distract an enemy (help action) to give you advantage on the attack rolls. (And you never get disadvantage again.)

3. You can use the familiar to deliver mail, as it will follow your orders directly, meaning it wont bite your finger off while tying mail to its leg, and it won't easily get lost. (Kind of like the owls in Harry potter)

If you have any other uses for find familiar, post them on the thread.

1) Find familiar does not say in sight, just unoccupied.   You can summon it on the other side of a door and look through it's eyes.  Technically you can summon a spider familiar inside a safe and examine the contents.

2) Send the familiar into that mysterious Gate with instructions to return in 1 round.  Whoops, it died.  Maybe do not enter the Gate marked "Abandon all hope, yea who enter here."

3) Load 5 Find Familiars into a Ring of Spell Storing and spend a day passing the Ring around for attunement and casting.    Every single member of your party has a Familiar now, which they can dismiss and re-summon without using a spell.  Only need to repeat the procedure when the familiar dies.

4) have your familiar deliver a touch spell to a prisoner who has lower health and may be an enemy of the state to finish them off with no one knowing what happened.

5)have your familiar be a constant lookout for night watches. since you and it share a telepathic link it can alert you to danger during the night.

6) Tell your DM that unless otherwise stated or you're indoors, your hawk is always 100 feet directly above you looking out for danger.

7) Need to go into a castle or any similar structure? Fully scout out the entire structure looking through the eyes of your rat or just look through the windows with your hawk. Using the familiar to scout like this is actually totally game breaking unless the DM specifically plans to counter it.

8) Starving? Nothing says you can't eat it, but it has to still be alive since it disappears at 0 hit points xD

9) Have your hawk carry a pebble 200 feet and then drop it on an enemy. An object or creature falling 200 feet takes 20d6 bludgeoning damage and deals half of that to the thing it lands on, so that's a nice, free 10d6 damage...

10) Put your familiar in the tank's pocket. Cast Cure Wounds on the tank through the familiar.

#8 is some hard core.  As a DM I would let you do it, but man, I would house rule all familiars are terrified of you.

# 9 does not work.  That is a thrown non-weapon, exceeding max range.  Auto-miss.   If you want to house rule against it auto-missing, expect the DM to house rule about the damage.

#10 I really like.  Pretty sure it should work

I don't want to number this, but one thing I want to advise is that you should see if your DM will allow you to use some of the more exotic options for Familiars. A Flying Monkey is probably the most versatile, as you can more easily justify it performing complex tasks like delivering potions to other players or untying knots or any number of other things. A Tressym is a flying cat that sees invisible objects/creatures, and can detect the presence of poison (which it's also completely immune to). Less exciting is an Almiraj, which doesn't have any super unique abilities, but it is a cute little rabbit with a little unicorn horn, and that's gotta count for something, right?

There are also creatures like Gazers and Pseudodragons, where their stat block mentions an optional rule for them to function as Familiars, but doesn't state that it functions as the Find Familiar spell... so just in case you're looking up lists of exotic familiars and you see them in the list, just know that they're not intended to function the same way (although I think they'd both qualify for a Pact of the Chain Warlock).

Thanks for all these ideas! I will definitely use a lot of them.

one more idea is to have the familiar steal something and then temporarily dismiss it and then make it appear in your hand, holding the item. (Only works if it has a way to hold onto it)

By RAW, when a familiar vanishes, any item worn or held by the familiar drops to the ground.  Your DM has house ruled that it appears in your hand.

Quote from Mog_Dracov >>

By RAW, when a familiar vanishes, any item worn or held by the familiar drops to the ground.  Your DM has house ruled that it appears in your hand.

 Ok. That makes sense, another way you could do it is just have the familiar sneak back to you after stealing the item (think Aladdins monkey)

I also came up with another use for find familiar, where you make the familiar look at you and switch your sight to the familiars sight then and use it like a mirror of sorts.

If you can't see, and you happen to have your familiar as an octopus, just make it grab your head and place its eyes over yours like a pair of goggles. 

Some of this is only achievable with the Warlock, but still...

Summon familiar outside the cell I'm locked in and it swipes the key to get me out.

Tie the guard's shoelaces together right before I start running away.

Drop a blanket/cloak over the head of an enemy.

Take your cookies out of the oven because you're still in the bath.

Snuff out all the torches up ahead while scouting to take advantage of our darkvision.

Make sounds off in the trees or trigger a trap so the person on watch in their camp is distracted long enough for me to sneak in and snatch some valuables.

Pretty much literally anything I can think of that Mage Hand, Unseen Servant, (a few Thaumaturgy/Druidcraft/Prestidigitation effects) might be able to accomplish.

Quote from Sanvael >>

6) Tell your DM that unless otherwise stated or you're indoors, your hawk is always 100 feet directly above you looking out for danger.

7) Need to go into a castle or any similar structure? Fully scout out the entire structure looking through the eyes of your rat or just look through the windows with your hawk. Using the familiar to scout like this is actually totally game breaking unless the DM specifically plans to counter it.

8) Starving? Nothing says you can't eat it, but it has to still be alive since it disappears at 0 hit points xD

9) Have your hawk carry a pebble 200 feet and then drop it on an enemy. An object or creature falling 200 feet takes 20d6 bludgeoning damage and deals half of that to the thing it lands on, so that's a nice, free 10d6 damage...

10) Put your familiar in the tank's pocket. Cast Cure Wounds on the tank through the familiar.

 The one issue with casting cure wounds is that that spell isn't on the same spell List as find familiar, meaning you need to take the ritual caster or magic initiate feat, or multiclass to a class that can cast find familiar.

Edit: ritual caster doesn't have cure wounds my bad

it is on a couple of the same lists. Druid burn a wild shape to cast ff and it has cure wounds and the celestial warlock that goes pact of the chain would also gain access to it.

Cast Dragon’s breath on your Owl familiar to have a firebrearhing owl with flyby  

Quote from Bazromark >>

Cast Dragon’s breath on your Owl familiar to have a firebrearhing owl with flyby  

 Omg that would be epic

2 words. Chronurgy... Wizard... At 10th level, they gain a feature called Arcane Abeyance where they can, once per short rest, store a spell of up up to 4th level in a small orb for an hour. Once this spell has been stored, requiring the wizard to 'cast' it (spell slot and ability feature consumed), any creature holding it can use its action to cast the spell within. It uses the chronurgist's spellcasting stats, but everything else considers the new creature to be the caster.

Familiars are this subclass' best friend... literally. If you plan on having a fight within the next hour, condense a strong (within previously mentioned limitations) concentration spell, hand it to your familiar, and let the combos begin. If you've ever wanted to execute a spell combo with two concentration spells, but haven't had the opportunity because you're the only caster in the party with access to these spells, you can now pull it off. For spells that buff yourself, but require concentration that can be broken upon attack, just have your familiar be the one to cast it and then get out of range of the enemies. As an example, you could have your familiar cast Polymorph on you, turning you into a tanky hunk of meat, then fly straight up. You can now contribute to the fight even if you're out of spell slots (given you prepared this ahead of time) and not risk death even if the wizard under the beast is actually at 1hp because the only way to get to that 1hp is to first drill through the hundred plus hp of your new form (or dispel magic, of course but the point is you won't lose concentration on it). It also has great use with some of the previously mentioned uses of familiars. Scouting can now become assassination or just be aided by an up to 4th level spell. Hand your familiar a spell like Wall of Fire and send them into the enemy warlords chambers while he sleeps. Let the chaos ensue. Even better, with the catnap spell you can feasibly give your familiar two saved spells for 50 minutes, one concentration and one non concentration, and it becomes its own unit of destruction in battle. This is all assuming your familiar can "hold" the item, so I typically do it with something like a hawk or owl.

Another aspect of this is the fact that you can give your whole party familiars, owls who can grant the barbarian advantage with the help action and flyby, or bats who can explore sketchy places with the rogue, maybe a squadron of hawks for the party to systematically search a given area, whatever the party requests. 

Quote from The_Mathlete >>

2 words. Chronurgy... Wizard... At 10th level, they gain a feature called Arcane Abeyance where they can, once per short rest, store a spell of up up to 4th level in a small orb for an hour. Once this spell has been stored, requiring the wizard to 'cast' it (spell slot and ability feature consumed), any creature holding it can use its action to cast the spell within. It uses the chronurgist's spellcasting stats, but everything else considers the new creature to be the caster.

Familiars are this subclass' best friend... literally. If you plan on having a fight within the next hour, condense a strong (within previously mentioned limitations) concentration spell, hand it to your familiar, and let the combos begin. If you've ever wanted to execute a spell combo with two concentration spells, but haven't had the opportunity because you're the only caster in the party with access to these spells, you can now pull it off. For spells that buff yourself, but require concentration that can be broken upon attack, just have your familiar be the one to cast it and then get out of range of the enemies. As an example, you could have your familiar cast Polymorph on you, turning you into a tanky hunk of meat, then fly straight up. You can now contribute to the fight even if you're out of spell slots (given you prepared this ahead of time) and not risk death even if the wizard under the beast is actually at 1hp because the only way to get to that 1hp is to first drill through the hundred plus hp of your new form (or dispel magic, of course but the point is you won't lose concentration on it). It also has great use with some of the previously mentioned uses of familiars. Scouting can now become assassination or just be aided by an up to 4th level spell. Hand your familiar a spell like Wall of Fire and send them into the enemy warlords chambers while he sleeps. Let the chaos ensue. Even better, with the catnap spell you can feasibly give your familiar two saved spells for 50 minutes, one concentration and one non concentration, and it becomes its own unit of destruction in battle. This is all assuming your familiar can "hold" the item, so I typically do it with something like a hawk or owl.

Another aspect of this is the fact that you can give your whole party familiars, owls who can grant the barbarian advantage with the help action and flyby, or bats who can explore sketchy places with the rogue, maybe a squadron of hawks for the party to systematically search a given area, whatever the party requests. 

 Wow that is a pretty great combo!

I found more uses for the spell:

1: if you have been swallowed by something, you can switch to your familiars sight using an action, and cast misty step as a bonus action (you must see where you are going to teleport before you can go there) you can also use this to plant a bomb or something around a corner, then misty step to safety.

2: if your familiar can fly, you can use it as a drone in real time.

Put your familiar in the backpack of your party's rogue. Now they always have an ally within 5 feet of an enemy they are melee attacking.

Bonus points if the rogue is also a kobold with pack tactics.

Quote from PeteInCary >>

Put your familiar in the backpack of your party's rogue. Now they always have an ally within 5 feet of an enemy they are melee attacking.

Bonus points if the rogue is also a kobold with pack tactics.

 This works. If you combine is with the elven accuracy feat and have your familiar use the help action, giving you triple advantage (3 dice to roll) this helps Rouges a ton because they only get 1 attack per turn, and it might as well hit, and do an absolute ton of damage.