I carry one ACEO that I made for myself with my daily words to live by. It's meaningful to me. I guess that means it's not technically an ACEO -- I wouldn't sell it for anything.
I also carry a small snack-size zip bag with a stack of unfinished ACEOs: some plain white ones and some with colored backgrounds. Plus, I carry a handful of pens and pencils. Sometimes I'll sit at lunch and doodle.
I also sometimes carry a finished ACEO with me. Sometimes I'll finish one and especially like it or make one that's completely different and gets me thinking about a new series or direction. I'll put it in a plastic sleeve and take it with me. I might put it on my desk at the day job, on the kitchen counter at home, or even on the dashboard in my car.
And, I have a little album with finished ACEOs that I sometimes carry with me so that I can show other people my cards. Friends will often ask what I'm up to, and it's handy to be able to show them my latest cards. Or, I'll take it out of my purse and leave it sitting on the table while I have coffee or lunch, and you'd be surprised how many conversations it starts. So I always tuck a few business cards in the back of it.
Do you carry ACEOs with you?