To celebrate Gamescom, GOG is offering The Witcher 2 for $29.99, the lowest price we've seen (read our performance review here). Meanwhile, Steam has compiled a bundle pack of Warhammer 40,000 games for $99.99 (or 50% off scattered titles), Impulse is serving Far Cry and Far Cry 2 for $14.99, GamersGate has 75% off Red Faction Guerilla and Killing Floor, and Direct2Drive cuts 60% off Magicka and Atom Zombie Smasher.

Steam
Warhammer 40,000 Complete Pack $99.99
Dawn of War Platinum Edition $14.99 (50% off)
Hoard $4.99 (50% off)
The Baconing $11.99 (20% off)
Multiwinia $3.40 (66% off)
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Impulse
Far Cry Bundle $14.99 (50% off)
World of Goo $4.98 (75% off)
Fate of the World $4.99 (50% off)
Cubetastic $3.99 (50% off)
Battle Slots $9.99 (50% off)
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GamersGate
Tropico 3 $7.48 (50% off)
Killing Floor $3.74 (75% off)
Dungeons $19.98 (50% off)
S.T.A.L.K.E.R. Clear Sky $4.98 (50% off)
Kane & Lynch 2: Dog Days $9.98 (50% off)
Red Faction Guerrilla $4.99 (75% off)
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Direct2Drive
Amnesia: The Dark Descent $7.95 (60% off)
Magicka $3.95 (60% off)
Atom Zombie Smasher $5.95 (60% off)
NightSky $4.95 (50% off)
Jamestown $5.95 (40% off)
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Games for Windows
Mass Effect 2 $14.99 (25% off -- also free when you preorder Battlefield 3)

Good Old Games
The Witcher 2: Assassins of Kings $29.99 (40% off)
40% off assorted titles