Celebrating the launch of its Linux client, Steam has discounted everything in its catalog that runs on the open source operating system, including half or more off Torchlight II, FTL: Faster Than Light and many other top indie releases. Elsewhere, you'll find decent savings on The Sims, Mass Effect and Civilization franchises as well as 66% off Max Payne 3. It's worth noting that although Steam offers Serious Sam 3: BFE for $7.99, you can pay about the same amount and get five extra games as part of Indie Royale's Valentine's promotion.

Steam
87 Torchlight II $9.99 (50% off)
85 Amnesia: The Dark Descent $5.00 (75% off)
88 FTL: Faster Than Light $4.99 (50% off)
78 Penumbra Collector Pack $3.99 (80% off)
77 Killing Floor $6.80 (66% off)
89 Bastion $3.74 (75% off)
85 Trine 2 $3.74 (75% off)
84 The Book of Unwritten Tales $4.99 (75% off)
90 World of Goo $2.49 (75% off)
61 Alien Breed Trilogy $4.59 (80% off)
86 Frozen Synapse Complete $8.74 (75% off)
77 Space Pirates and Zombies $2.49 (75% off)
87 Superbrothers: Sword & Sworcery EP $3.99 (50% off)
75  Serious Sam 3: BFE $7.99 (80% off)
84  Crusader Kings II $9.99 (75% off)
Dwarfs!? $3.40 (66% off)
Hacker Evolution Duality $4.99 (75% off)
Castle Crashers $7.49 (50% off)
The Amazing Spider-Man $19.99 (60% off)
Counter-Strike: Source $4.99 (75% off)
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GameStop
88 Battlefield 3 $19.99 (50% off)
78 Tropico 4 $4.99 (75% off)
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GamersGate
87  Mass Effect 3 $9.98 (50% off)
93  Mass Effect 2 $4.99 (75% off)
90  Mass Effect $4.99 (75% off)
83  Sonic Generations $7.49 (50% off)
87 Civilization V GotY $12.49 (75% off)
94  Civilization IV Complete $6.24 (75% off)
Civilization III Complete $2.48 (50% off)
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GameFly
75 Dead Island GotY $6.79 (66% off)
73 Spec Ops: The Line $7.49 (75% off)
77 The Darkness II $7.49 (75% off)
69 Payday: The Heist $4.99 (75% off)
55 Duke Nukem Forever $4.99 (50% off)
82 Mafia II $7.49 (75% off)
75 Ghost Recon: Future Soldier $11.99 (60% off)
NBA 2K13 $14.99 (50% off)
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Green Man Gaming
84 Max Payne 3 $13.59 (66% off)
84 Max Payne 3 + Rockstar Pass $17.49 (75% off)
82  Chivalry: Medieval Warfare $12.49 (50% off)
75  Doom 3 BFG Edition $14.99 (50% off)
77  Lego Lord of the Rings $14.99 (50% off)
Guns of Icarus Online $6.79 (66% off)
Scribblenauts Unlimited $14.99 (50% off)
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Get Games
66 Dungeons Gold Edition $7.49 (75% off)
78 Endless Space Admiral Edition $14.99 (50% off)
64 Disciples III Gold Edition $7.49 (75% off)
68 Dungeon Siege III $4.99 (75% off)
63  Jagged Alliance: Back in Action $9.99 (75% off)
81  Tropico 3 $3.39 (66% off)
Grotesque Tactics $4.99 (75% off)
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Amazon Digital
50-66% off The Sims titles

GOG
50% off RPGs