the old cs which knowing valve if it excedes CS:S in the stats they might unleash another bad update upon it, css is 5,000 people down from where it was yesterday. http://store.steampowered.com/stats/
Then theres the obisidian conflict mod which will be getting steam support with the next update in 2012:
Then some of the games I havent played in quite some time in my steam profile: quake 3 arena, evil genius and il2, been meaning to get a joystick for it for quite some time.
and then theres resistance and liberation and black mesa if they ever come out.
I've played obsidian conflict and it was pretty fun. Its just that half of the time non english speaking people are playing it.