Assuming you can break away from Diablo III and Max Payne 3, there are plenty of deals to be had this weekend, including steep discounts on Darksiders, Syndicate, The Darkness II, both recent Batman games, Mafia II, Left 4 Dead 2, Just Cause 2, Sanctum, Mirror's Edge, SimCity 4, the first three Assassin's Creed titles, Far Cry one and two, as well as virtually everything in Paradox Interactive's arsenal. Also of note, the Humble Indie Bundle has added three more titles (Lone Survivor, Braid and Super Meat Boy) to what was already an awesome bargain.
Steam
{pf id:2969 size:small} Magicka Collection $6.24 (75% off)
{pf id:2543 size:small} Hearts of Iron 3 Collection $14.99 (50% off)
{pf id:3394 size:small} Penumbra Collector Pack $9.99 (50% off)
{pf id:1608 size:small} Cities in Motion Collection $19.99 (50% off)
{pf id:2976 size:small} Majesty 2 Collection $9.99 (50% off)
{pf id:2895 size:small} The Lord of the Rings Online Steam Starter Pack $14.99 (50% off)
{pf id:83183 size:small} King Arthur Collection $24.99 (50% off)
{pf id:83592 size:small} Crusader Kings II Collection $24.99 (50% off)
{pf id:3120 size:small} Mount & Blade Collection $17.49 (50% off)
{pf id:85214 size:small} Warlock - Master of the Arcane $9.99 (50% off)
The Kings' Crusade Collection $9.99 (50% off)
Crusader Kings Complete $4.99 (50% off)
Hearts of Iron 2 Complete $7.49 (50% off)
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Amazon Digital
{pf id:81115 size:small} NBA 2K12 $9.99 (50% off)
{pf id:37424 size:small} Medal of Honor $15.00 (25% off)
{pf id:3645 size:small} Rift $5.58 (72% off)
{pf id:38282 size:small} Tropico 3 $4.41 (56% off)
{pf id:54210 size:small} Tropico 4 $19.99 (50% off)
SimCity 4 Deluxe Edition $14.77 (26% off)
Crysis Maximum Edition $8.99 (70% off)
Major League Baseball 2K12 $9.99 (67% off)
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GameStop
{pf id:1815 size:small} Darksiders $4.99 (75% off)
{pf id:1251 size:small} Assassin's Creed $4.99 (75% off)
{pf id:1252 size:small} Assassin's Creed 2 $9.99 (50% off)
{pf id:36360 size:small} Assassin's Creed: Brotherhood $14.99 (50% off)
{pf id:37158 size:small} Homefront $9.99 (50% off)
{pf id:1339 size:small} Battlefield 3 $39.99 (33% off)
Far Cry Bundle $4.99 (75% off)
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GamersGate
{pf id:83611 size:small} Syndicate $20.40 (66% off)
{pf id:4174 size:small} Chronicles of Riddick Assault on Dark Athena $3.74 (75% off)
{pf id:3086 size:small} Mirror's Edge $6.78 (66% off)
{pf id:80502 size:small} Sengoku $4.50 (50% off)
{pf id:2747 size:small} Just Cause 2 $5.98 (60% off)
{pf id:2747 size:small} Just Cause 2 Complete Edition $10.00 (60% off)
{pf id:80954 size:small} Renegade Ops $6.00 (70% off)
{pf id:3758 size:small} Sanctum Collection $6.80 (66% off)
{pf id:2824 size:small} Lead and Gold: Gangs of the Wild West $1.50 (50% off)
Just Cause $5.20 (60% off)
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GameFly
{pf id:83503 size:small} The Darkness II $12.49 (75% off)
{pf id:81334 size:small} Batman: Arkham City $7.49 (75% off)
{pf id:1316 size:small} Batman: Arkham Asylum GotY $4.99 (75% off)
{pf id:3514 size:small} Prototype $4.99 (75% off)
{pf id:2954 size:small} Mafia II $7.49 (75% off)
{pf id:2831 size:small} Left 4 Dead 2 $6.79 (66% off)
{pf id:1532 size:small} Call of Duty: Modern Warfare 2 $9.99 (50% off)
SimCity 4 Deluxe Edition $4.99 (75% off)
Grand Theft Auto IV $4.99 (75% off)
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Green Man Gaming
{pf id:81012 size:small} Nuclear Dawn $12.47 (50% off)
{pf id:82579 size:small} Jagged Alliance: Back in Action $19.99 (50% off)
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GOG
50% off Anuman titles