I've used a lot of usernames over the years. Probably the first one, which I used to use on BBSes in the early 90s, was "Diceman". Like the George Cockcroft (Luke Rhinehart) book, but in my case it was a school nickname because back then I collected dice.
Much later I became "Scatman Dan" in many circles, after Scatman John, after some new friends from university discovered that I knew all of Scatman John's lyrics by heart.
But by the time you've shed your anonymity online, the horse has already bolted and there's little use shutting the stable door. So nowadays, I just go by my real, full name basically everywhere. If you see a "Dan Q"... it's probably me... or else some annoying Dan Quayle or Dan Quigley or similarly-named person who's chosen to use just the initial of their surname with no regard for those of me whose surname only has a single character in the first place, grr.
Upside: I'm very easy to find, because my name is short and unique. Downside: the same thing.
tl;dr: it's my actual name