Along with free copies of Dragon Age: Origins and Bejeweled 3 on EA's platform, Borderlands 2 is only $5 at GameStop or $6.80 if you'd rather shop at GamersGate, which also lists Tomb Raider and Saints Row: The Third - The Full Package for $5 each, Hitman: Absolution for $6.25, Skyrim for $9.98 and Thief for $10.20. Also of note, the latest Humble Bundle includes both Darksiders releases alongside 10 other titles for $10 or whatever you're willing to part with.

Steam
75 Serious Sam Complete Pack $9.99 (90% off)
71 Naruto Shippuden: Ultimate Ninja Storm 3 Full Burst $9.99 (67% off)
83 Wargame Franchise Pack $20.00 (60% off)
76 Divinity Anthology $7.50 (75% off)
64  Contrast $3.74 (75% off)
72 Blood Bowl: Chaos Editon $7.49 (75% off)
Space Engineers $11.99 (40% off + free weekend access)
Kerbel Space Program $17.99 (40% off)
Race The Sun $2.49 (75% off)
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GameStop
87 Borderlands 2 $4.99 (75% off)
87 Borderlands 2 GotY $13.59 (66% off)
90 Company of Heroes $2.99 (66% off)
74 Cities in Motion 2 $4.99 (90% off)
85 Europa Universalis IV $9.99 (75% off)
43 Sword of the Stars II Enhanced Edition $4.99 (75% off)
78 Victoria II $4.99 (75% off)
75 Magicka $2.49 (75% off)
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GamersGate
87 Borderlands 2 $6.80 (66% off)
87  Borderlands 2 GotY $20.00 (50% off)
88 Tomb Raider $5.00 (75% off)
79 Hitman: Absolution $6.25 (75% off)
73 The Incredible Adventures of Van Helsing 1 Complete $5.00 (75% off)
83 Saints Row: The Third - The Full Package $5.00 (75% off)
93  TES V: Skyrim $9.98 (50% off) 
93 TES V: Skyrim Legendary Edition $20.00 (50% off)
82 Just Cause 2 $3.75 (75% off -- DLC pack $1.75)
68 Thief $10.20 (66% off)
Legacy of Kain: Soul Reaver $1.50 (75% off)
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GameFly
87 Sid Meier's Civilization V Complete $19.99 (60% off)
89 Portal $2.49 (75% off)
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Green Man Gaming
84 Age of Empires III Complete Collection $9.99 (75% off)
FortressCraft Evolved $4.49 (50% off)
80% off Youda games
Up to 85% off Mastertronic games
20% off select titles via "OCTOBE-R20GMG-OFFERX"
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Get Games
66  Painkiller Hell and Damnation Collectors Edition $7.49 (75% off)
Blade Symphony $7.49 (50% off)
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Origin
90 Dragon Age: Origins $0.00 (free)
82 Bejewled 3 $0.00 (free)
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GOG
Pick three of these games and get up to 85% off