Most likely it's the crow whistle and gaming dice.

Never know when you may need a crow, or to roll for something, and they don't take up much space in my purse.

My ex-wife got me an engraved Zippo I used to carry everywhere, now it's mostly on my desk because I don't want to lose it at work :/

Not really unique, but I started carrying small Bluetooth earphones on my keyring. They are excellent for listening to audiobooks etc when doing solo boring tasks like food shopping etc.

Pink plastic heart and a marble in my pants' change pocket. My daughter gave them to me on two separate occassions when she was 2-3. She said "this is your heart" and "this is your sould" when she gave them to me and they've been in my change pocket almost every day for the last 5+ years.

I work in RF and have to mess around with SMA cables all the time. As a result I carry a 5/16 wrench on my lanyard.

I have the wallet of an old German conductor out of München Gladbach, Arnold Queasten. I found it in a thrift store as it looked like a nice quality leather wallet.

In it are still some notes from Arnold, on two of his business cards. A note of someone here knew from the war, and another person from the flute section.

There was one pfennig, a German penny still in it. So I have Arnold's last penny. I found his grave on findagrave, one day I'll visit that.

I'm now terribly afraid of losing the wallet, though, because I'm unsure a casual finder would appreciate the beauty of such a piece.

I've got a black joker card in my wallet. I was walking to the tram stop with someone once, when I saw this playing card face-down on the floor, so I said "bet you I can guess this card," I knelt down, and I said "I think it's a joker." I was about to turn it over when my friend said "hold on, black or red?" I said black and turned it over, and now it lives in my wallet.

A set of keys that will open most of the doors in a certain national park. Turns out “government use only. Do not duplicate” doesn’t mean a whole lot to a crooked locksmith. My plan is to go test and see if they still work when I’m an old man. I won’t be surprised if they do

A tiny vial of peppermint clove oil. I struggle with sleepiness and lack of focus so after I've had an inadvisable amount of caffeine and am still not functional, I dab a bit right under my nostrils to help me wake up and focus a bit better

Also works as mosquito repellent on some species so if I forget bug spray I use some of this instead

I used to have a 1/4" nut that missed the threading process. I had it on my keychain to remind me that even if something is done millions of times without error, sometimes mistakes can happen.

Lost it when my backpack was stolen.

A pack of nicotine gum.
I don't need or use it anymore, but I still carry it with me every day.
It's a reminder of how addicted I was and how I swore to never smoke another cigarette in my life.

Keychain fingernail clippers (Amazon)

I’ve had the same pair (different brand than linked) for many years and they have held up great. It’s crazy how often they come in handy, and my SO asks to borrow them all the time too. They are extremely small and nearly weightless. Highly recommended.

Edit: Found the exact brand I have. True Utility. Not sure if it’s worth the slightly higher price since it’s the only ones I’ve ever had – can’t compare. But again, they’ve held up great for many years.

I have a keychain with four or five mini crochet hooks.

I used to carry a universal TV remote control with me. It's simply staggering how often it was useful.

The Key.

Small high power flashlight, about the size of a tube of m&ms. People laugh at it all the time, but it comes in handy weekly, sometimes daily.

Think of how many times you pull out your credit cell phone light. Now imagine being able to light up a room like daylight if need be. Very useful.

Those same people who laugh, eventually realize the value, and come asking for my light when we're together and it's needed

Not necessarily unique, but I mostly always carry a small pouch containing a full D&D dice set.

I haven't needed dice in... years, now that I think about it, but I carry them as a symbol of luck. Not good luck or bad luck, just luck, chance. Helps me keep things in perspective when everything goes sideways.

My fingerprints.

I keep ear plugs in my zip pouch wallet so I'm always ready for any concerts or random loud areas.

Made a 5" aluminum baseball bat in high school shop class learning how to use a lathe. It's been on my keys ever since. Thing is probably older than most folks reading this.

Edit: pic as requested. :) https://i.imgur.com/jrxStc4.jpeg

My granfather's watch.

A crow-calling whistle and a small tin of peanuts.

A metal red uno reverse card sits in my wallet. It was a gift from my girlfriend and an acknowledgement of our in joke of replying no u to compliments and I love yous.

Syringe of thermal paste

Multitool

Leaving aside points about driving licence numbers being unique or whatever, it would be the silver pentagram that I made back in the '90s and have worn (or occasionally carry in my wallet etc, when the cord breaks) ever since.