Frankly, I don't understand how someone can possibly own Half Life Two, and not own the downloadable Episode 2, but if you fit that category, you're in luck, because Steampowered.com is selling Episode 2 for 80% off. That makes it two bucks, and needless to say, it's a must buy if you already have Half Life 2 and Episode One, not to mention an excuse to grab the lot of them if you haven't played this franchise yet.
Direct2Drive.com has a few things going on today only (Tuesday the 22nd). They're doing a Daily Deal on games by Sega all week long, and today it's Football Manager 2011 for 40% off which makes it $23.95. D2D also has two dollar tuesdays and today you can get Westward, Fishdom, or Space Giraffe for two dollars each.
GamersGate.com is doing an Atari Week with many games by Atari on sale for around 60%. This includes some great deals like Chronicles Of Riddick: Assault on Dark Thena for eight dollars, and classic sci-fi RTS game Total Annihilation for less that four dollars. They also have a few other games with assorted deals as well.
ImpulseDriven.com has a daily deal on Men of War for $3.74, but they also a whole bunch of simulator games on sale too for about half off (Prices vary).
The BEST DEAL OF THE DAY goes to Direct2Drive.com for their giraffe-tastic sale on Space Giraffe.