You've undoubtedly heard about Team Fortress 2 going free-to-play, but in case you missed the news, here it is again: Team Fortress 2 is now free! Valve has accompanied that bombshell with a proper weekend sale, cutting 50% off Homefront (though, based on its mediocre reviews, this might be the catalyst to a permanent price drop). Folks looking for some old school action should check out GamersGate's deals on Deus Ex and Thief, while Direct2Drive has steep discounts on more recent hits including Civilization V, Mafia II and Borderlands.

Steam
Homefront $24.99 (50% off)
Star Ruler $5.00 (75% off)
Europa Universalis III Complete $3.75 (75% off)
Europa Universalis III Chronicles $7.50 (75% off)
Team Fortress 2 (now free -- permanently)
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Impulse
Shift 2: Unleashed $24.99 (50% off)
Tom Clancy's Ghost Recon: Advanced Warfighter $9.99 (50% off)
Cities in Motion $13.99 (30% off)
50% off Aveyond titles
50% off King's Bounty titles
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GamersGate
Deus Ex Collection $9.98 (50% off)
Thief: The Dark Project and Deadly Shadows $12.45 (50% off)
Magicka $5 (50% off)
Lara Croft and the Guardian of Light $3.74 (75% off)
25 to Life $5 (50% off)
Conflict: Denied Ops $7.48 (50% off)
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Direct2Drive
Battlefield: Bad Company 2 $7.95 (60% off)
Up to 75% off 2K titles (includes Civ V, Mafia II, Borderlands, and BioShock 2)
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Games for Windows
BlazBlue: Calamity Trigger $17.49 (50% off)

Good Old Games
50% off Interplay titles