While there's plenty of great deals on offer this weekend, please don't overlook the charity-supporting Humble Indie Bundle (Pay What You Want). As the name implies, you get six games for however much you're willing to spend, and you can even tweak how much of your total purchase goes to charity and/or the developers themselves.

Steam
• Call of Duty: Modern Warfare 2 - Free Multiplayer Weekend
• Civilization III Complete - $1.25 (75% Off)
• Civilization IV: The Complete Edition - $10 (75% Off)
  • Civilization IV - $5 (75% Off)
  • Civilization IV: Warlords - $2.50 (75% Off)
  • Civilization IV: Beyond the Sword - $5 (75% Off)
  • Civilization IV: Colonization - $7.50 (75% Off)
• Shattered Horizon - $14.99 (25% Off)
• Shattered Horizon Four-Pack - $44.99 (25% Off)

Direct2Drive
• Age of Conan - $9.95 (50% Off)
  
  Direct2Drive Spring Sale Week 4
  • Mass Effect 1 + 2 Bundle Pack - $34.95 (50% Off)
  • Aion - $24.95 (50% Off)
  • Titan Quest Bundle Pack - $4.95 (75% Off)
  • Vampire: The Masquerade - Bloodlines - $9.95 (50% Off)
  • Sid Meier's Pirates! - $4.95 (75% Off)
  • Civilization IV: Complete Edition - $9.95 (75% Off)
  • SimCity Societies - $9.95 (50% Off)
  • SWAT 4 Gold - $9.95 (50% Off)
  • Fallout 3 - $14.95 (50% Off)
  • Sacred 2: Fallen Angel - $19.95 (33% Off)
  • Divinity II - $29.95 (40% Off)
  • Chessmaster 9000 - $4.95 (50% Off)
  • Defense Grid: The Awakening - $4.95 (50% Off)
  • Majesty 2: $8.95 (70% Off)
  • The Settlers: Rise of an Empire Gold - $9.95 (66% Off)
  • Trine - $4.95 (75% Off)
  • Guild Wars: Nightfall - $14.95 (50% Off)
  • Grand Ages Rome - $9.95 (66% Off)
  • AI War / Zenith Remnant Bundle - $9.95 (67% Off)
  • Europa Universalis: Rome Gold - $4.95 (75% Off)
  • Dark Falls Lost Souls - $4.95 (83% Off)

Impulse
• Arsenal of Demoncracy: A Hears of Iron Game - $13.99 (30% Off)
• Flotilla - $6 (40% Off)
• Immortal Defense - $4.99 (50% Off)
• SimCity Societies - $9.99 (50% Off)
• Tropico 3 / Abosulte Power Add-On Pre-Order - $42.49 (25% Off Tropico 3)
• Warhammer 40K: Dawn of War Gold - $9.99 (50% Off)
• World War 2 - Time of Wrath - $22.49 (25% Off)

Good Old Games
• Constructor - $4.19 (30% Off)
• Creatures Exodus - $4.99 (50% Off)
• Creatures Village - $4.99 (50% Off)
• Creatures: The Albian Years - $4.99 (50% Off)
• Mob Rule - $4.19 (30% Off)

Get Games
• NecroVision: Lost Company - $14.99 (66% Off)
• Rig'N'Roll - $13.95 (20% Off)
• Space Rangers 2: Reboot - $6.66 (66% Off)

The Charity-Supporting Humble Indie Bundle (Pay What You Want)
• Aquaria
• Gish
• Lugaru HD
• Penumbra Overture
• World of Goo

Frictional Games
• Amnesia: The Dark Descent (Pre-Order) - $10 (50% Off)

Ubisoft
• Buy One, Get One Free (Select Titles)

GamersGate
• Codename: Panzers Cold War - $19.958 (50% Off)
  Up to 80% off depending on "memeber status."
  
• And Yet It Moves - $4.97 (50% Off)
• Achtung Panzer: Kharkov 1943 -
• Cannon Strike - $9.97 (50% Off)
• Combat Mission: Shock Force - $4.97 (50% Off)
• Desert Thunder - $4.99 (50% Off)
• Elliptic Twist - $2.95 (25% Off)
• Frontline: Fields of Thunder - $4.99 (50% Off)
• King Arthur: The Role-Playing Wargame - $19.97 (50% Off)
• Military Life: Tank Simulator - $11.47 (50% Off)
• O.R.B. - $9.97 (50% Off)
• Panzer Elite Action Gold - $9.97 (50% Off)
  • Panzer Elite Action: Fields of Glory - $4.97 (50% Off)
  • Panzer Elite Action: Dunes of War - $4.97 (50% Off)
• Panzer Elite Special Edition - $2.47 (50% Off)
• Panzer Killer! - $9.99 (50% Off)
• PikGem - $9.97 (50% Off)
• Tank Universal - $4.97 (50% Off)
• Thandor: The Invasion - $2.47 (50% Off)