SpookMeister - because I once needed to come up with a new name for myself in a FPS that I was into (AmericasArmy). My teamates were always talking about how playing against me was like playing with a ghost... they never knew where I'd be coming from and I scared the crap out of them all the time. Well, there was already someone who went by Ghost(something or another) so I went with Spook.

Spook (and variations of same) are fairly common on-line. So I ended up adding a Meister to it. I still get the occasional "already in use" but it works more often than not.