Well looking at my steam list I can spot:
Blood Bowl - we were trying to get a leauge going on this but people lost intrest.
Killing Floor - quite a few have it and play it on and off.
L4d1/2 - Used to have fun on vurses but people kept being poor sports and rageing which ruined it for some.
TF2 - public evry now and then can be nice and tf2lobby.com is a low stress alternitive to pickups.
Alien swarm - its free most people played it to death on launch but im sure it could be fun evry now and then.
Battlefield bc2 - again a few have it but seems to be out of favor atm.
cs/css - smells

Guild wars - at one point we had loads of people into it again intrest dwindled.
Moonbase alpha - my pet fetish even if u just twat about on it its fun.
Obsidian conflict - always nice to play farm map once in a while and maybe other maps if people get intrested in it.
Titan Quest - diablo style hack and slash that again people played alot at one point.
trackmainia nations forever - free against the clock raceing game the harder tracks make for hillarity.
Zero Gear - cart style raceing gamew with some intresting game modes cheep to get and owned by many already.
Other things I personally like
Diablo2, borderlands, civ 4(would say 5 but i havnt got it yet!) , freelancer , supream commander , worms , dungeon siege , age of empires 3 and MASTER OF ORION 2!!!
also a nod to the free to play mmo's