In addition to Dead Space still being free at Origin, Steam offers free weekend access to Frozen Synapse and the fifth Humble Moble Bundle has arrived with six titles including The Cave for $5 or whatever you're willing to pay. Steam has also steeply discounted Saints Row releases, Arma II content, Lego titles and more, GamersGate is serving up deals on Tropico and Civilization, GMG has launched a promotion with Thief for $17 among other bargains and Get Games has 75% off Borderlands 2 and BioShock Infinite.

Steam
81 Kingdoms of Amalur: Reckoning Collection $11.99 (70% off)
82 Saints Row IV $13.59 (66% off)
83  Saints Row: The Third - The Full Package $5.99 (80% off)
77 Lego Lord of the Rings $4.99 (75% off)
76 Arma II Complete Collection $9.99 (75% off)
82 Lego Batman 2 DC Super Heroes $4.99 (75% off)
82 Beat Hazard Mega Bundle $8.49 (50% off)
86 Frozen Synapse Complete Pack $6.99 (80% off + free access weekend)
System Shock 2 $4.99 (50% off)
Cortex Command $9.99 (50% off)
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GamersGate
78 Tropico 4 Gold Edition $7.99 (80% off)
81  Tropico Trilogy $3.99 (80% off)
94  Civilization IV Complete $12.48 (50% off)
Civilization III Complete $2.48 (50% off)
Dominions 3: The Awakening $4.99 (75% off)
Final Exam $5.00 (50% off)
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GameFly
78 Sonic & All-Stars Racing Transformed $4.99 (75% off)
83 Sonic Generations $7.49 (75% off)
78 Train Simulator 2014 $19.99 (50% off)
20% off select titles via "APR20OFF"
Castle of Illusion $4.99 (67% off)
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Green Man Gaming
69  Thief $16.99 (66% off)
74  Typing of the Dead: Overkill $6.79 (66% off)
80  Sleeping Dogs $4.99 (75% off)
76 Rage $4.99 (75% off)
72  The Cave $3.74 (75% off)
88 Tomb Raider GotY $7.49 (75% off)
38 Damnation $2.49 (75% off)
81 F1 2013 $12.49 (75% off)
78 Grid 2 $7.49 (75% off)
79 Hitman: Absolution $6.24 (75% off)
58 Krater Collector's Edition $4.99 (75% off)
Dirt Showdown $7.49 (75% off)
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Get Games
88 Borderlands 2: GotY $9.99 (75% off)
92 BioShock Infinite $7.49 (75% off)
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GameStop
88  The Walking Dead $6.24 (75% off)
77  Back to the Future: The Game $6.24 
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GOG
60% off select EA titles