I thought of making this thread, because of my current campaign. I'm playing a young, female, goliath genie warlock (currently level two at the time of writing this), with the staff of the python, named Jörmungandr. 

Tia is a part of a mercenary tribe, and the tribe has a marid as their otherworldly patron.  Her mother, before Tia left to do a mission her father asked her to do, asked the patron for, "An item that will protect my daughter, but also be a source of companionship on her lonely travels".

Despite being told that the "item" is meant to protect her, Tia only sees Jörmungandr as an actual sentient creature. She'll feed him, play with him, and would always say that he "Loves hugs". 

One of my homebrew items is a sentient flail inhabited by the spirit of its former wielder's dog. The user didn't want to part with his faithful companion, and so had the dog's spirit bound to his weapon, so it would always be by his side.

I don't have my notes on me at the moment, but among its properties are that the flail's handle wags when something exciting is happening. It likes to fight new kinds of monsters, so any encounter with something new will have the flail's head pull toward it like a dog on a leash.

It responds to "weapon-y" names like Macey.

I have a hexblade warlock who's patron is his sword, a nine lives stealer. It acts like the normal item in all ways, except that it houses his patron's soul, and she can communicate with him telepathically. The basic premise is that after her death, she was forced to wander the land as a ghost due to a curse by the gods. A powerful wizard came across her ghost and bound it inside a sword, transforming it into a weapon of deadly power. Years later, my character broke into the house of the sword's current owner and stole it, hoping to sell it for money. After the sword revealed it was sentient, they realized that their goals were aligned, and the spirit taught him magic. His two main goals are to free his patron from the sword and to get revenge against the gods (he has a grudge against them, too).

I have a Draconic Bloodline Sorcerer character with a dagger that has a mind of it's own.  The ancestor whose spark is what gave her that power was called Blaze, and he had a dagger named "Ardor".  The dagger has been handed down in the family over the generations.  It has no bonus to hit, does 2 points of elemental damage and can cast Chaos Bolt once per day.

I figured that would be fine for a low level Sorceress.  Her ancestor is a Gold Dragon, and he's still around.  Someday Ardor will help her find him.  At 14th level she can grow wings, and off they go to find him.  At level 20, she becomes a dragon herself, and can be the one who gives that bit of blood for another Sorcerer someday.

Bickerblade.  I'll sketch the concept rather than impose the stats.  Scale it how you like, it's an intelligence weapon, lawful neutral in alignment.  Give it language based magic like comprehend languages, command, etc.  Make it a powerful weapon, +2 or more _BUT_ it only engages well in combat or avails its user of its powers  if Bickerblade is also engaged in a compelling argument with its wielder. It may even halt an attack if provided a compelling argument by its target (though common sense applies it would put itself in complete jeopardy).  Bickerblade doesn't need to win the argument, and rather would face a lengthy challenge than something that can be resolved simply.  If it doesn't feel intellectually engaged, it's bonuses are reversed or just nonexistent (your determination).  Probably best suited for a College of Eloquence Bard.  Arguments can be either role played or performed via INT/WIS/CHR skill checks depending on the topic.

I haven't figured out what it would be beyond concept, but Bickerblade of course has a nemesis blade, chaotic neutral/chaotic evil leaning The Absurdist.  Tasha's Hideous Laughter will be available to it, not sure what it's gimmick will be, maybe require the wielder to perform increasingly ridiculous actions approaching entropy in combat.  I suppose it could be "big cursed" in that it's wielder could become enchanted by its maniacal meaninglessness until the wielder fully descends into nihilism and become unplayable unless rescued by the forces of reason.  Psychic damage effect as the wielder gets caught up in the joke?

Knight (Eventide), don't read this if you're on here.

I have a Hexblade warlock whose patron is unknown to him, but he has a trinket item that is a greatsword blade hilt that has been broken off from the blade. This character and his sister have some extremely heavy ties into the Shadowfell. In the long-term, this is going to play out. In the short-term, I've given him [Tooltip Not Found] with some custom background flair that fits with my worldbuilding. I don't plan for this to be an endgame item for the character, although the player seems to be acting and now even building around that as an idea.

When it comes to sentient items, I think the most important thing to consider is that item's agenda. It will want to work through the owner/PC, and might enact consequences on them if that character fails to fulfil their goals.

I made a sentient sword for a Paladin in a previous campaign, she found it in a Troll's Lair where they were worshipping it as a god, which really bothered the sword as it was created to destroy evil.  It was intended to be a 'singing' sword, but a crafting mistake caused it to be a 'stuttering' sword instead. However, when the paladin used her divine abilities through the sword (smite and channel divinity) the sword was able to speak without the stutter for a few moments.

Solodôs was crafted in the fashion of Elven singing swords by a well-meaning human smith. Inscribed with powerful runes that give the blade an intelligence and a drive to vanquish evil, an error in translation also causes Solodôs to speak with a pronounced stutter.

60' vision, speaks common with a stutter
INT 16, WIS 14, CHA 9, Alignment: LG
Greatsword +1 if wielder Lawful or Good, +2 if both

Once per day can cast Slow (DC 16) on a single evil target that it hits.
Once per day can cast Confusion (DC 16) on a single chaotic target that it hits.
Abilities reset at dawn.

Quote from MysterX >>

I made a sentient sword for a Paladin in a previous campaign, she found it in a Troll's Lair where they were worshipping it as a god, which really bothered the sword as it was created to destroy evil.  It was intended to be a 'singing' sword, but a crafting mistake caused it to be a 'stuttering' sword instead. However, when the paladin used her divine abilities through the sword (smite and channel divinity) the sword was able to speak without the stutter for a few moments.

 

Solodôs was crafted in the fashion of Elven singing swords by a well-meaning human smith. Inscribed with powerful runes that give the blade an intelligence and a drive to vanquish evil, an error in translation also causes Solodôs to speak with a pronounced stutter.

60' vision, speaks common with a stutter
INT 16, WIS 14, CHA 9, Alignment: LG
Greatsword +1 if wielder Lawful or Good, +2 if both

Once per day can cast Slow (DC 16) on a single evil target that it hits.
Once per day can cast Confusion (DC 16) on a single chaotic target that it hits.
Abilities reset at dawn.

 Not going to lie, that's pretty  cool. Imagine if the sword actually converted the trolls? XD

I had an idea for a sentient weapon intended as a player character for a player who can't consistently attend. It would have a class and gain levels, but it couldn't use class abilities itself, only activate one each round for the player wielding it.

I have a silly one called the Introverted Blade.

It gives +2 to attacks, and +3 if outdoors. It constantly mutters excuses and reasons why it would be best if it returned to its scabbard, and if outdoors these turn to near panicked ravings about how this is lovely but it really must get home and feed the cat, and it might have left the stove on, and it's getting towards bed time and it really should get home.

If you have it drawn on any turn and don't make an attack, it flies back into its scabbard and requires a strength check to remove it again, which is higher if it's outdoors!

One of the players in my campaign (started as a Swashbuckler rogue) has a book that began at level 1 as A Tattered Old Fencing Manual. That incarnation it worked as follows:

Swift to the point: If you spend one hour studying this manual and practicing its techniques during a long rest, you may add +1 to one attack roll of your choice after the long rest has completed. The attack must made with a short sword, longsword, rapier or scimitar. You must declare that you are using this bonus before making the attack roll, after which it is expended whether the attack hits or not.

As time went on, the book communicated with him in dreams, and the pages began to appear bloody as though a bloody blade had been wiped across them. He began feeding the book blood from slain enemies, and around level 3 the book changed as follows:

This book, written partly in common and partly in infernal, appears to be a fencing manual, teaching a style of fencing you have never encountered before. Interspersed with the fencing techniques are paragraphs describing the lives of diabolical fencing masters who put their students through cruel sessions. The opening paragraph seems to be a discussion about whether these techniques would work "in a real fight."

The Impenetrable Art of Defence: If you spend two hours studying this manual and practicing its techniques, you learn two manoeuvres of your choice from among those available to the Battle Master archetype in the fighter class for 7 days. You gain two superiority dice, each of which is a d6 (this die is added to any superiority dice you have from another source). This die is used to fuel your manoeuvres. A superiority die is expended when you use it and you only regain your expended superiority dice by feeding the book (see below). You may change the manoeuvres  you know by studying the tome for a further 2 hours. If a manoeuvre you use requires your target to make a saving throw to resist the manoeuvre's effects, the saving throw DC equals 8 + your proficiency bonus + your Strength or Dexterity modifier (your choice).

Curse: The fencing manual must be fed the blood of an enemy by spending an action to wipe a bloodied blade across a page. The blood must come from a hostile creature of CR1 or more than you have participated in killing. When you do so, you regain all lost superiority dice 1 hour later.

Obviously this is pretty powerful! But...

As time has gone on, the character has learned that the book is in fact a legendary demon named Shraak'Nizaar, who devours the souls of those who become bound to it. The book trains them to get stronger and stronger, until Shraak'Nizaar feels that they present a worthy challenge. The book is also indestructible; when they tried to burn it, it reappeared back in the rogue's pack within an hour. He cannot be rid of it unless the characters find a way to destroy it. The book has already consumed the rogue's former fencing master, and hundreds of other lost souls. The characters managed to dream into the book and took on ancient warriors there. 

This will is all part of an epic quest chain, which will require them to discover the only way to be free of it.

I love the idea of a demon/other-powerful-creature giving power to mortals, exclusively to make them a worthy opponent! I'm definitely stealing that one!

That's pretty interesting. 

Alternative name: the shut-in sword. XD

I was actually in the process of designing a "Sentient Weaponsmith" subclass for the Artificer!

One of the features included infusing your sentient weapon with a distinct "personality quirk", chosen from a table:

1. The weapon is extremely judgmental of the wielder's martial prowess; praising success & scrutinizing failure.

2. The weapon has a bloodthirsty personality; and will suggest violence at nearly every opportunity.

3. The weapon likes to be cleaned regularly; desiring to be polished until it is gleaming, and dislikes rust, grime & other dirty substances.

4. The weapon prefers to be wielded by the race of it's maker...and may believe the wielder is of that race, regardless.

5. The weapon may shout out sound effects, curses, or narrate itself while in combat.

6. The weapon may become jealous if it's current wielder uses another weapon, and plot revenge...against that weapon.

7. The weapon is terrified by combat, and will shriek, scream, or otherwise beg for mercy when faced with a hostile opponent.

8. Every time the weapon lands a critical hit; the language it can speak changes.

Quote from Tony1Adobe >>

I was actually in the process of designing a "Sentient Weaponsmith" subclass for the Artificer!

One of the features included infusing your sentient weapon with a distinct "personality quirk", chosen from a table:

1. The weapon is extremely judgmental of the wielder's martial prowess; praising success & scrutinizing failure.

2. The weapon has a bloodthirsty personality; and will suggest violence at nearly every opportunity.

3. The weapon likes to be cleaned regularly; desiring to be polished until it is gleaming, and dislikes rust, grime & other dirty substances.

4. The weapon prefers to be wielded by the race of it's maker...and may believe the wielder is of that race, regardless.

5. The weapon may shout out sound effects, curses, or narrate itself while in combat.

6. The weapon may become jealous if it's current wielder uses another weapon, and plot revenge...against that weapon.

7. The weapon is terrified by combat, and will shriek, scream, or otherwise beg for mercy when faced with a hostile opponent.

8. Every time the weapon lands a critical hit; the language it can speak changes.

 

 Number 6 sounds like the weapon will be a Yandere. XD