I assume you already know the big ones so I'm going to recommend some smaller F2P titles I've enjoyed. Path of Exile is diablo-style game that got an update recently, you can play alone or in a party. Boid is a simple RTS game that I think needs more attention.
If you don't know them, the big ones are TF2, CS:GO and DOTA 2 - very very popular Valve games exclusive to Steam, but I've only played DOTA 2 (I have 1000+ hours in it, but from beta and first year of release).