For me, it's the Killing Floor, I get it for free in the Humble Bundle.
What is the best free steam game you got?
I don't know if it was actually on Steam back then, but the original America's Army. I had more hours in that than any other game I've played (aside from WoW). I got into the closed beta for AA:PG and played that for a while until moving. Still haven't put my gaming rig back together/played the final release.