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.

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

Bag of folding.

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

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.

That’s half the fun! Sometimes, the true clown you spawn is yourself.

Jesus…

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)

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.

I can’t see any of these working as intended. Clowns don’t subscribe to reality

Spawn them as a distraction!

Some of them cause a big distraction that accidentally points directly towards those you don’t want to be seen.

Use them to hide!

One of them will look giant and big to hide you while the others honk and gesture/point behind, clearly showing where you are.

Build yourself a mountain of clowns to scale any wall!

Crabs in a bucket. None will let you climb. You must stay to hear their jokes…

Never starve again with their endless supply of pies!

Shaving cream pies. Ain’t nobody got time to bake 30 coconut creams

Use their weight to bring down any air-/ship

They all blow up helium balloons to help it float. Unless you want it to float in which case their balloons turn into bowling balls at the last second with a big shrug.

Air drop them on their enemies!

See balloons

Just crush your entire party by spawning hundreds of them in a closed room!

Clown car logic. You’re all “crushed” but it’s just extremely difficult terrain.

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.

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

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).

Never starve again with their endless supply of pies!

Probably bad that without the parentheses, I was already assuming this was some kind of horrific Sweeny Todd situation.

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.

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

Or makes you smell horrible. Yes I just stole this from ‘cruelty squad’.

The periapt of health also keeps you from the fun effects of drugs (including alcohol); not useless, but keeps working for 24 hours after you take it off

Deck of many uncomfortable things Normal mix of good and bad, weak and powerful, but as an example: the two dueling demon lords it spawned aren’t fighting for power or honor; this is a domestic dispute between two extremely intimate (succubus intestines can do what? Why? WHY?) supernaturally-dickish immortals who have been emotionally abusing each other for a thousand years, and they’re airing all their dirty laundry. Take sides if you dare.

Ring of mind shielding: Also shields you from anything uncomfortable; you do not gain xp. (Yeah yours is better)

Ring of mind shielding (extremely racist): you can’t see elves’ faces now. you’re welcome.

Portable hole: It’s soft, meaty, and contains multiple (always closed) giant sphincters. Sometimes shudders. Otherwise functions normally. So far.

Wand of wishes, but gives you what you actually want right now.

Yeah, you need a bridge, but you’re fatigued, so on some level what you really want is a hot bath and a warm bed; maybe some soup.

But people can see your footsteps cartoon style

Wand of Wonder What This Does

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