My real name is Adam Wiseman, the 303 came from a site I tried to sign up where wiseman was taken, and wiseman303 was one of the alternate nicks the site suggested. I liked the way it looked, and have used it for almost everything since then.
The story of my Half-Life/TFC nicknames is a little more interesting.
I started with "Atomic", because I couldn't think up anything better at the time.
I was -]vise[- for a while because I saw someone's nick surrounded by =[ ]=, and it reminded me of a vise, so with some modifications I made it my nickname.
Then I was "Atomic Newbie" for a while, Atomic from my original nick, and Newbie because I wasn't very good.
As time went on, I developed a habit of priming a grenade and holding it while rushing in for close combat. This tactic usually killed me as well as my victim, so I became "Atomic Kamikaze."
I eventually got bored with Atomic Kamikaze, and I had just started learning C++ programming, so my nick became "Newbie++"
When I'm having an off day, and getting killed more often than not, I change my name to "Newbie--" which usually makes people laugh, and helps me not take the situation too seriously.
I will probably change my name again in the future, maybe even resurrecting Atomic Kamikaze (Kamikaze++?), but I'm happy with Newbie++ for now.
Oh yeah, I was IM>U for a very short time somewhere in there, until I saw someone called I_M>U.
Cheers everyone