GAMES: 
(M)[4] = Multiplayer with the number being the rating of multiplayer out of 5 in my opinion
==============
Steam:
---------
Alien Swarm (M)[3.5]
Aquaria
Arma II (M)[2.5]
AudioSurf
Batman: Arkham Asylum
Bioshock
Bioshock 2 (M)
Blacklight: Tango Down (M)[3.5]
Borderlands (M)[3.5]
Braid
Jericho
Company of Heroes (and all expansions) (M)[4]
CounterStrike: Source (M)[4]
Darwinia
Day of Defeat: Source (M)[3.5]
Dawn of War II (All Expansions) (M)[3.5]
Defcon (M)[3]
Dragon Age: Origins
Evil Genius
Far Cry 2 (M)[4]
Foreign Legion: Buckets of Blood
Garry's Mod (M)[varies]
Ghost Master
Ghostbusters
Gish
Global Agenda (M)[1.5]
Grand Theft Auto IV (M)[2.5]
Half-Life 2
Half-Life 2: Deathmatch (M)[4]
Hegemony: Philip of Macedon
IL-2: Sturmovik (M)[2.5]
Imperium Romanum
King Arthur (M)[2.5]
Left 4 Dead (M)[3.5]
Lego Star Wars: The Complete Saga (M)[4]
Lugaru HD
Machinarium
Magic: The Gathering (M)[3]
Mass Effect
Mount & Blade: Warband (M)[3.5]
Multiwinia (M)[2.5]
Osmos
Overlord
Overlord II
Penumbra: Overture
Portal
Red Faction: Guerrilla (M)[3.5]
Rome Total War: Gold Edition (M)[4]
Civilization III (M)[4]
Civilization V (M)[4.5]
Sin
Sin Episodes: Emergence
Sniper Elite
Team Fortress 2 (M)[4.5]
Thief: Deadly Shadows
Splinter Cell: Conviction (M)[haven't tried]
Torchlight
TrackMania: United (M)[3]
Uplink
World of Goo
X3: Reunion
X3: Terran Conflict
Zombie Driver (M)[haven't tried]

Direct 2 Drive:
---------
Civilization IV (All Expansions) (M)[5]
Dirt 2 (M)[4.5]
Men of War (M)[3]
Titan Quest (All Expansions) (M)[2.5]
X-Com (All Games)

GOG:
---------
Arcanum: Of Steamwork and Magick Obscura
Jagged Alliance: Deadly Games
Stronghold (M)[3.5]
Tyrian 2000

GamersGate:
---------
City Life 2008
Heroes of Annihilated Empires (M)[3]

Boxed Copies:
---------
007: Nightfire (M)[2]
Age of Empires (M)[4.5]
Age of Empires: Rise of Rome (M)[4.5]
Age of Empires II: Age of Kings (M)[4.5]
Age of Empires II: Conquerors (M)[4.5]
Age of Mythology (M)[4.5]
Age of Mythology: Titans (M)[4.5]
Battlefield 2 (All Expansions) (M)[4]
Battlefield 2142 (All Expansions) (M)[4]
Battlefield: Bad Company 2 (All Expansions) (M)[4.5]
Battleforge (M)[2]
Burnout: Paradise (M)[3]
Call of Duty 2 (M)[4]
Call of Duty 4: Modern Warfare (M)[4]
Call of Duty: World at War (M)[haven't tried]
Command and Conquer: The First Decade (M)[3-4.5]
Dawn of War (All Expansions) (M)[4.5]
Dawn of War II (All Expansions) (M)[4.5]
Deus Ex Game of the Year (M)[3.5]
Deus Ex: Invisible War
Dune 2000 (M)
Enemy Territories: Quake Wars (M)[2]
Fallout 3
F.E.A.R. (M)[3]
Gothic 3
GRID (M)[3.5]
Indiana Jones and the Emperor's Tomb
Just Cause
Kill Switch
Lord of the Rings: Battle for Middle-Earth (M)[3]
Mirror's Edge
Need for Speed: Most Wanted Black Edition (M)[3.5]
Need for Speed: Carbon Collector's Edition (M)[3]
NOX (M)[3]
Oblivion: Game of the Year Edition
Prince of Persia: Warrior Within
Quake 3 (M)[3]
Rainbow Six: Lockdown (M)[3]
Rainbow Six: Ghost Recon Advanced Warfighter 2 (M)[haven't tried]
Red Faction II (M)[2.5]
Return to Castle Wolfenstein
Rise of Nations (All Expansions) (M)[4]
RollerCoaster Tycoon (All Expansions)
Serious Sam 2 (M)[haven't tried]
Sins of a Solar Empire (Bj's) (M)[2]
Sims
Sim City: 3000
Skateboard Park Tycoon 2004
Splinter Cell: Pandora Tomorrow
Splinter Cell: Chaos Theory (M)[haven't tried]
Spore
StarCraft (All Expansions) (M)[4]
StarCraft 2 (M)[4.5]
Starwars: Battlefront II (M)[3.5]
Starwars: Dark Forces
Starwars: Galactic Battlegrounds Saga (M)[2.5]
Starwars: Jedi Knight: Dark Forces II (M)[4]
Starwars: Jedi Knight: Jedi Outcast (M)[4.5]
Starwars: Jedi Knight: Jedi Academy (M)[4.5]
Starwars: Knights of the Old Republic
Starwars: Rebellion (M)[3.5]
Timeshift (M)[3.5]
Tomb Raider: Underworld
Tribes: Vengeance (M)[3]
TZAR (M)[2]
Unreal Tournament: Game of the Year (M)[4.5]
Unreal Tournament 2003 (M)[3]
Unreal Tournament 2004 (M)[4.5]
Unreal Tournament 3 (M)[3]