Pretty sure it was my first steam game. Don't remember if I bought it on release week or after though.
It was THE first Steam game, if I remember Steam launched the same day HL2 was released or slightly before.
I'm pretty sure Steam was out before that, as I remember having no end of problems trying to play CS through it.
When HL2 came out it "re-launched" though.
HL2 was the first game to require Steam client to play.
My GOD!!!! What did you do with all that space?