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 why I swore to never smoke another cigarette in my life.

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.

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

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.

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

Multitool

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

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 granfather's watch.

Syringe of thermal paste

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.

Probably not super unique but a glass breaker/seatbelt cutter. If I'm in an accident or I come upon one, I want that ability with me.

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.