Try one in your world today

Scroll of summon wisp.

When used nothing appears, but you gain a speech impediment for 1d6 days.

Bag of Communal Holding

Content is shared with all other bags of communal holding in existence. Sometimes retrieving objects involves awkward hand contact if someone else is using their bag at the same time.

A potion of True Healing... heals 1d8+2 damage, recipient MUST truthfully answer the next question they are asked. Sell the characters 6, but don't tell them about the truth serum. Let them figure it out on their own.

Boots of Elvenkind... except Elves can hear you.

A bag of holding that contains infinite clowns. Every time it is opened, 1d4 clowns come out. The clowns are useless in combat and attempt to distract, annoy and mock the holder. While this could be used as a distraction, the clowns will follow the holder, drawing attention to them. You could create a table for what kind of clowns you get (mime clowns, pie throwing clowns, balloon animal clowns, magician clowns, etc). The clowns will wander off after 1d6 minutes. Where the clowns go and what they are (Illusions? Demons?) is unknown.

I have two I will be using in my next campaign:

Ring of attunement: Provides 1 extra attunement slot. (Requires attunement)

Event Staff: This staff allows the wielder to gain unquestioned entry into any "employees only" areas or zones otherwise off-limits to the public. Anyone (including actual staff or other officials) who sees the wielder in one of these areas will assume they are a known employee or other official who is granted special access to the area. Unfortunately, they will all also view the wielder as the least competent and least trustworthy employee or official with the organization. Any actions taken in the area are likely to be closely watched and highly scrutinized by any observer who would know better.

ETA: One from the current campaign in which I am a player character. Our DM thought of this one:

Bullet of Healing:

This magical bullet can be loaded into any firearm. Whomever is shot by this bullet first receives 1d6 piercing damage followed by 1d10 healing. If the initial damage causes recipient's HP to fall below 0 before the bullet's healing effects begin, they will fall unconscious and will not gain any healing effect from the bullet. Instead, one death save is automatically passed.

Bag of folding.

A bag of holding except anything stored in it comes out folded in half.

Potion of Water Breathing: DOES NOT RETAIN AIR BREATHING

Sir Mix-a-Lot (unrelated) is a traveling potion salesman who shows up for my party at suspiciously specific times, and generally has discounted potions specifically tailored for whatever they happen to be doing at the time. For example, if they need to be really strong, he'll have a bottle of Sir Flex-a-Lot's Magical Muscle Maximizer, which does increase the strength of one's muscles, but not of their bones or connective tissue (it was designed to be used only in bodybuilding competitions), so whenever the drinker does a STR check, they must also make a CON saving throw to avoid breaking a bone or tendon. Need to decipher an ancient text? Try Sir Scripts-a-Lot's Polyglottal-in-a-bottle, which will let you read unknown languages, but also comprehend all unknown languages, even those of the plants and animals around you, making it very difficult to concentrate on any one thing. (inspired by https://www.smbc-comics.com/comic/springtime)

Ring of invisibility. Makes you invisible, but makes everyone else invisible to you as well.

Ring of protection. Grants everyone around you protection in a fairly large radius. Might be useful for long range combat, maybe. Might also be useful to navigating certain environmental hazards.

Boots of Flying. They can fly, but only have a carry weight of a few pounds. If you're more than say ten pounds, the little wings flap but gain no altitude. They are not autonomous. Might be useful in condunction with other magics to reduce weight.

Gauntlets of Ogre Might. Do not affect strength. They do tell you the odds of nearby ogres taking particular actions. They might do this, they might do that, and so on.

Hammer of Striking. Social bonuses when organizing labor. Combat bonuses only when near many allies.

Boots of Haste. Gain extra actions but large penalties to all checks. Haste makes waste. May be useful if combined with large bonuses or fixed outcomes (eg: DND diviner wizard).

Sweaty Sword:

Really good sword, strong steel. The handle is wrapped in living leather harvested from the palms of a cursed pervert. It's always slightly warmer than your hand and it exudes a sticky substance that enhances grip. Smells like corn.

Survival Stew Balls:

A fried ball of...food. It's rock hard, slightly too big to hold with one hand easily, completely impermeable, and covered in a flaky, delicate panko breaded crust. To eat, boil one in 5 gallons of water to produce a pot of stew. The flavor is different for every ball. Never cook two in the same pot at the same time. Wash the pot thoroughly within 6 hours after removing from heat. especially if it's made of iron. Under no circumstances are you to reheat the left overs.

Emergency Shews:

Bubble gum that turns into one time use sandals. Once the flavor runs out, you have 30 seconds before the gum expands into shoes. The sizes seem to have been printed on the gum but they've long since faded or rubbed off. Durian flavor.

Dead Cat Bounce:

A black bottle with a cat eye painted on it. If you drink the contents and die due to falling from a great height, you will be revived immediately and launched with equivalent force in a random direction.

Cape of (Refugee) Flight: you gain the power to fly for your life.

Screaming Cloak of Invisibility: you're invisible, but the cloak constantly screams, "HE'S OVER HERE!!!" and tries to give away your location.

The Tax Axe: raises both your taxes and your target's with every swing.

Sword of charisma: But it attracts bugs for some reason.

Amulet of protection: Stops heals too.

Boots of speed: Brakes not included.

Wand of light: Slightly radioactive.

Potion of restoration: Removes buffs as well, kills undead.

Wand of Fireball: has a medium chance of shooting a stream of cinnamon whiskey.

Cube of instant castle: Say the keyword 'open' to transform this cube into a '200x'200 castle. The transformation happens instantly, and if you're caught in the area of effect, be prepared to get smashed. The cube is hard of hearing.

Helm of Invincibility: you are invincible, but only to people who have Vince in their name, e.g. Vince, Vincent, some other name with Vince

Periapt of health: this is a small vial with a red liquid inside attached to a small chain. While wearing this, all diseases that you would otherwise contract enter the small vial instead. If the vial is broken, the closest creature will immediately contract all diseases contained within.

Deck of many things (used): the previous owners of this deck got all that they could have wished for. The remaining cards might not be the best.

Ring of mind shielding: the creator of this ring was a bit over-zealous. Along with the usual effects, this ring will censor violence, sex, and other uncouth things.

Cloak of the bat: along with the usual effects, wearing this cloak will also make you speak bat. You will only be able to produce high-pitched squeaks.

Portable hole: this portable hole is bottomless! Anything that falls down the hole is lost forever.

Bag for holding:

It's a bag of holding but instead of occupying a bag slot it must be kept in the main hand.

Band of Gorilla Repair: Once per day, can repair anything, or rather, will summon 1d4 (can be modified depending on the size of the job) massive gorillas who show up seemingly out of nowhere whenever anything near the wearer breaks or is heavily damaged. The gorillas can repair anything.

Those not expecting to see a bunch of repair-happy gorillas must make a fear check.

These mysterious gorillas are actually friendly and fix whatever thing was broken, but beware, their patience quickly runs out for anybody intentionally causing disrepair or destruction in their presence!

Yeah, it works just like this!

Leaky alchemy jug.
Whenever you use this jug roll a d100. This is how many percent of your choosen liquid are replaced by mayonnaise.

Arrows of Accuracy: These arrows will always hit. Starting from the target, then out from that point, the hit roll is checked against defenses.

If nobody is hit, the arrow strikes the firer.

Bag of holding with a preexisting hoarding problem

Can’t-trip - only works when you don’t want it to

Book of spells, but you have to spell out letter by letter each time you cast

Circlet of human perfection, but the creator had a really niche fetish.

Bag of Storing, a bag of holding that doesn’t mitigate the weight of the items within.

I have a spreadsheet full of these somewhere. The 2 that my players got that I remember them using are:

Emperors armor: +3 full plate armor with no strength or armor proficiency requirement. When you look at yourself or in a mirror you see yourself in full plate armor, but to everyone else you are naked and if anyone tells you that you are naked the armor and all your possessions cease to exist. My player made it the shopping district in the heart of the city before someone told him.

Staff of disintegrating: when activated, it disintegrates. The player that got this one saved it for a boss fight. He found it hilarious though.

Not quite the same, but a Paladin in a campaign I was in once bought a Shield of Missile Attraction for cheap because the shopkeep thought it was cursed.

Wand of wishes. It's great, but it will respond to any speaker in the vicinity

Bag of holding, but everything that goes in comes out a crocheted plushie version.

Bead of (Uncomfortable) Nourishment: Standard effect, but must be taken nasally. Oral absorption may cause permanent magical flatus.

Sword with fire enchantment, fire has no regulator and cannot be turned off.

Wand of Wonder What This Does

A regular wand of wonder that comes with a gambling addiction.

Broom of Flying Yes its a broom that allows you to fly

No one ever said anything about landing The broom cannot come down lower than 30 feet from the ground. Dismounting will stop the broom and allow you to pick it up, as long as your concious from the fall

Flawed bag of holding - it's a bag of holding, but the dimensions inside are normal bag sized.

Movable rod - an immovable rod, but it can only resist up to 10lbs

Bed of comfort - it's comfortable, just not the way you were hoping.

Vorple blade - it's exactly what it is, but it's too dull to cut and sharpening it would damage the enchantment.

Immovable Rod: Only one end of the rod is fixed in space, the other end swings freely.

Caltrops of Seeking. Drawn to metal and increase chance to damage/hit metal boots.

They stick together though and must be manually placed one by one over an extended period of time.

I gifted my party a sapient dagger. It was really good too. It was a 1d6 but let you attack again, but rolling a 5 or less on the die makes you insane because the dagger insults the user so badly for missing. If it happened 12 times, the character died. They used it from level 3 or 4 through the end of the campaign at level 18, since there was no cap on how often you could attack again.

Headband of miner intellect:

• Forged by a dwarven artificer who lost too many colleagues due to their lax attitude around safety.
• Anyone wearing this headband becomes aware of the quality of air and props for whatever tunnel they're currently in.
• Wearers possess an intrusive urge to find proper protective equipment before attempting a task.

Sword of ghostly might: One owned by a powerful warrior who came back as a vengeful spirit. Neglects to mention that the sword is also a ghost, and therefore can only deal damage to spirits.

"Wizards with Guns" did a couple of sketches based on this concept.

Cloak of Levitation, but it just lets you float a foot off the ground without moving.

Hulk Strength, but dumber than Hulk. You smash anything in the area, including your supplies/allies/horse...

Cap of Invisibility, but you can't see anything either.

Immovable Rod:

• Not actually a magical rod.
• Actually just a bloke called Roderick, or Rodney, or something to that effect.
• Won't move out of your way.
• Not even if you ask politely.

Wand of Premature Eruption: Launches a fireball that detonates immediately

The "Sword of Instant Regret". It does 100% DMG, but only stabs the wielder.