Ugh, way too many. Quite a lot of which I don't even really play and should probably get rid of. Not counting games which have seperate single-player and multiplayer applications there are 33:
Age of Chivalry (Half-Life 2 mod)
Aion
Audiosurf
Battlefield 2142
Dark Messiah: Might and Magic
Doom 3
EVE Online
Fate/stay night
Galactic Civilizations II: Gold Edition
Galactic Civilizations II: Twilight of the Arnor (never worked, fucking Impulse...)
Guild Wars
Kagetsu Tohya
Left 4 Dead
Left 4 Dead 2
Mount&Blade
One ~To the Radiant Season~
Painkiller Gold Edition
Perfect World International
Portal
Sacrifice
Saya no Uta
The Secret of Monkey Island
Starcraft
Supreme Commander: Forged Alliance
Team Fortress 2
Tribes Vengeance
Tron 2.0
Universe at War: Earth Assault
Utawarerumono
Warcraft II
Warhammer 40,000: Dawn of War - Soulstorm
X-com: UFO Defence
Yume Miru Kusuri
Damn, that's getting close to one for every letter of the alphabet! Let's see, I need a C, H, I, J, N, Q, R, V and Z. So if I installed Company of Heroes, Homeworld II, Nexus: the Jupiter Incident, the R.U.S.E beta and Vigil: Blood Bitterness I'd be almost there. Q and Z have me a bit stumped though...