Back when I started getting involved in online listservs and forums (for RPGs, at first), I figured I was 20 years older than most of the other posters, who tended to pick names of their favorite characters (in the game, or movies, TV, etc.) or other things that sounded "cool" to them. I didn't feel a need to sound cool. I couldn't think of anything clever, didn't really think adopting the identity of a published character was all that original, and just decided, the hell with it, I'm not going to worry about how anonymous I am.
So I've almost always used a variation of my real name. Boring, I know.
On the other hand, it means occasionally I get fan mail, because people who read things I wrote ten and twenty-plus years ago can find me. So that part ain't bad.