The only things I've got that can really be considered online is Maplestory, Trackmania Nations Forever, and NFS Underground 2, the latter which I play the least.
PC
Team Fortress 2
Garry's Mod
Counter Strike: Source
Audiosurf
Warcraft III
PS3
Resistance: Fall Of Man
Resistance 2
LittleBigPlanet
Rock Band 2
MotorStorm
MotorStorm: Pacific Rift
Call Of Duty 4: Modern Warfare
SOCOM: Confrontation
Grand Theft Auto 4
Guitar Hero 3: Legends Of Rock
Gran Turismo 5: Prologue
-pc-
CS:S
Left 4 Dead
Warcraft 3 - all expansions
Diablo 2 - all expansions
Starcraft - all expansions
WoW - Exodar Server
Aion - Ariel Server
Killing Floor
Team Fortress 2
Sins of a Solar Empire
-PS3-
Resistance 2
Killzone 2
Unreal 3
Resident Evil 5
Blazblue
Farcry 2
Wolfenstein
Little Big Planet
Street Fighter 4
Fear 2
Orange Box
SOCOM: Confrontation
Grand Theft Auto 4
will have soon
Ninja Gaiden Sigma 2
modern warfare 2 -prestige edition!!
Demon's Soul's - i want this game!!!!!!!
also i have a headset for both but i normally don't use it cause i like to listen to music or podcast while i play... i even play FPS games with no sound lol
CSS
HL2DM
Jedi Knight 2
Warcraft 3
Dungeon Keeper 2
Left 4 Dead
TF2
Dawn of War and all expansion except Soulstorm
Dawn of War 2
and i play Aion on Telemachus (EU) if anyone wants to join me
Heroes of newErth
left 4 dead not on steam, I play on garena
I still have to install my CS condition zero
--PC--
CS: Condition Zero (6.1)
AoE 1 and 2 (2 with expansion packs)
Quake 2
Urban Terror
Quake 3
Open Arena
PC
Starcraft
Warcraft 3 (someone mentioned Dota previously. know how to play that too)
UT2004
UT (the original)
Call of Duty
Call of Duty 2
Guild Wars
AoE2 (just download that)
CS 1.6
NFS Underground
... not going to bother listing the stuff that can't be played online.
PS3
... can go to hell
I vote Starcraft, and CS:S.