I came up with mine when I was trying to dign up on a forum realted to airsoft, which was also the first time I ever signed up onto a forum. Joker was taken, so I replaced the o with a 0, and it's stuck ever since.
I've always loved the movie Full Metal Jacket, and remembered the main character, who's nickname was Joker. The way he had "Born To Kill" written on his helmet, and wore a peace symbol on his jacket reminded me of myself, how I believe that war is sometimes nessecary, not to mention I am fascinated by war and how it affects people, and how its fought, but prefer for there to be peace in the world (like that'll ever happen ;) ). Not to mention I just like making contradictions to myself and others :P ! Then of course there is also the fact that I like joking around, and can find the humor in anything (although I do shutup if it's inappropriate for the moment).
[EDIT]
I forgot to ask some of you, how do you change your alias?