Although Steam's holiday event has expired, the service has a 50% debut discount on Age of Empires III (Microsoft seems to be gradually pushing its titles on Valve's platform). If you're still hunting for mega sales, GameStop and GamersGate have deals on some top titles, including Dead Space 2, Mafia II, Red Faction: Armageddon, The Witcher 2 and Super Meat Boy. It's also worth noting that Indie Royale has a New Year Bundle with Nuclear Dawn and three other titles available for about $4.
Steam
Age of Empires III Complete $19.99 (50% off)
Lead and Gold: Gangs of the Wild West $4.99 (50% off)
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GameStop
{pf id:1848 size:small} Dead Space $9.99 (50% off)
{pf id:36702 size:small} Dead Space 2 $9.99 (50% off)
{pf id:2954 size:small} Mafia II $7.49 (75% off)
{pf id:54210 size:small} Tropico 4 $19.99 (50% off)
{pf id:38282 size:small} Tropico 3 $9.99 (50% off)
{pf id:3589 size:small} Red Faction: Armageddon $14.99 (50% off)
{pf id:81987 size:small} Need for Speed: The Run $39.99 (20% off)
{pf id:38435 size:small} Zeit2 $4.99 (50% off)
{pf id:81561 size:small} Stronghold 3 $39.99 (20% off)
{pf id:2241 size:small} FIFA 2011 $19.99 (33% off)
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GamersGate
{pf id:2764 size:small} King Arthur Collection $10.18 (66% off)
{pf id:38297 size:small} Two Worlds II GotY $24.98 (50% off)
{pf id:2940 size:small} Machinarium $4.98 (50% off)
{pf id:80319 size:small} Dead Island Four Pack $89.85 (55% off)
{pf id:81116 size:small} Orcs Must Die! $7.50 (50% off)
{pf id:2037 size:small} Dungeons Mega Pack $16.48 (50% off)
{pf id:3913 size:small} Sniper: Ghost Warrior Gold Edition $11.98 (60% off)
{pf id:40478 size:small} The Witcher 2: Assassins of Kings $19.98 (50% off)
{pf id:4089 size:small} Super Meat Boy $3.73 (75% off)
Neverwinter Nights Diamond $4.98 (50% off)
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Direct2Drive
{pf id:37158 size:small} Homefront $7.50 (75% off)