In the event that you haven't downloaded it yet, Dead Space is available for free through Origin. There's no word on how long it will be available so don't wait if you're interested. Elsewhere, Steam has Sanctum 2 for a buck and change, Far Cry 3 is $7.50 at Amazon, GameStop has cut half off Rocksmith 2014 and Assassin's Creed IV, while The Walking Dead with its DLC is $7.49 and Borderlands 2 is $9.98 at GamersGate. Also, the latest Android/PC Humble Bundle expires in four days.

Steam
81 Sanctum 2 $1.49 (90% off)
81 Divinity: Dragon Commander $13.59 (66% off)
68 Sniper Elite V2 $7.49 (75% off)
68 Sniper Elite Franchise Pack $9.99 (75% off)
82 Deponia Trilogy $16.31 (66% off)
72 Alice: Madness Returns $4.99 (75% off)
90 Spelunky $5.09 (66% off)
55 Journey of a Roach $2.99 (80% off)
73  Velvet Assassin $0.74 (85% off)
Worms Revolution $5.09 (66% off)
Daedalic Entertainment Bundle $49.99 (77% off)
More...

Amazon Digital
85 Rainbow Six Vegas 2 $2.50 (75% off)
79 Battlefield 2142 Deluxe Edition $0.99 (93% off)
82 Splinter Cell: Conviction $5.00 (75% off)
89 Far Cry 3 $7.50 (75% off)
85 Far Cry 2 $2.50 (75% off)
More...

GameStop
86  Rocksmith 2014 $29.99 (50% off)
85  Assassin's Creed IV Black Flag $29.99 (50% off)
78 Tropico 4: Collector's Bundle $9.99 (75% off)
Sine Mora $2.49 (75% off)
More...

GamersGate
87 Borderlands 2 $9.98 (50% off)
89 The Walking Dead Pack $7.49 (75% off)
74 Ghost Recon: Future Soldier $4.99 (75% off)
83 AI War Four Year Anniversary Edition $4.25 (75% off)
Poker Night 2 $1.25 (75% off)
More...

GameFly
82 Hotline Miami $4.99 (50% off)
Foul Play $3.75 (75% off)
20% off select titles via "APR20OFF"
More...

Green Man Gaming
69 Q.U.B.E. $2.49 (75% off)
63  Alien Breed Trilogy $5.74 (75% off)
Superfrong HD $2.49 (75% off)
Skull Girls $5.09 (66% off)
{pf id: size:small} 20% off select titles via "9NXY7B-749QGZ-QIM2NG"
More...

Get Games
45 Aliens Colonial Marines $4.99 (75% off)
72 The Cave $3.74 (75% off)
71  Red Faction Collection $14.99 (75% off)
67 Kane and Lynch 2: Dog Days $2.49 (75% off)
64 Binary Domain Collection $5.74 (75% off)
More...

GOG
60% off Atari and Kalypso titles