If you've been hesitant to invest in the latest version of Modern Warfare, Steam removes cash from the equation with a free multiplayer weekend (along with a 33% discount, if you enjoy the ride). Valve's distribution house also cuts 75% off 2009's sequel to Return to Castle Wolfenstein and 25% off some classic GTA titles. There are plenty of deals elsewhere, including Civilization V and Mafia II for $7.49 each, Borderlands for $4.99, as well as Human Revolution and Space Marine for $14.99 each.

Steam
83 Call of Duty: Modern Warfare 3 $40.19 (33% off + free weekend)
72 Wolfenstein $4.99 (75% off -- today only)
Grand Theft Auto 3 $7.49 (25% off)
Grand Theft Auto: Vice City $7.49 (25% off)
Grand Theft Auto: San Andreas $11.24 (25% off)
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GameStop
87 Sid Meier's Civilization V $7.49 (75% off)
87 Deus Ex: Human Revolution $14.99 (50% off)
83 Borderlands $4.99 (75% off)
90 Plants vs. Zombies $4.97 (50% off)
82 Mafia II $7.49 (75% off)
82 Bejeweled 3 $9.99 (50% off)
71 Rainbow Six: Vegas 2 $19.99 (50% off)
X-COM Pack $2.99 (75% off)
BioShock $4.99 (75% off)
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Amazon Daily Digital
86  Battlefield: Bad Company 2 $4.99 (75% off)
75  Magicka Collection $4.99 (75% off)
87  Burnout Paradise $4.99 (75% off)
82  Dungeon Defenders $2.99 (80% off)
90  Mass Effect $4.99 (75% off)
93  Mass Effect 2 $4.99 (75% off)
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GamersGate
D&D Neverwinter Nights Complete $10.18 (66% off)
Neverwinter Nights 2: Platinum $6.78 (66% off)
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GameFly
75 Warhammer 40,000: Space Marine $14.99 (50% off)
88 Battlefield 3 $44.99 (25% off)

Green Man Gaming
83 Saints Row: The Third $39.99 (20% off)
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Games for Windows
88 Fallout 3 $9.99 (50% off)

Good Old Games
50% off Age of Wonders