Steam is having something of an off day. Their big Midweek Madness sale is Darkspore for 40% off. That makes it $29.95; not really "Madness", more like a perfectly reasonable price. Asking fifty bucks for the thing was the true madness.
Direct2Drive.com has a couple of sales going on right now. First is Homefront for $39.95. That's 20% off the regular price. Unfortunately the developer that makes it, Kaos Studios, is getting shut down by THQ, and your forty bucks probably won't save anyone's job. D2D is also doing Atari week, with a new game on sale everyday. At the moment, it's Ghostbusters: The Video Game for ten dollars. The older one that was pretty good, NOT the recent (And terrible) Sanctum of Slime.
It's Future Wars Week at GamersGate.com and you can get sixteen different sci-fi war games for 75% off. This means you could get Tom Clancy's Endwar for five dollars among a few other deals. Gamer's Gate also has around twenty other games on sale too; do take note that Killing Floor is down to ten dollars.
The Telltale Games Store is still doing their month-long sale on "Almost Everything" at 75% off. This include t-shirts, and physical merchandise too. The only exceptions being Back To The Future and Puzzle Agent 2 (Which is marked down by 10% as a pre-order bonus).