Steam is selling the indie game Terraria for half off at $4.99 until tomorrow night, and their Daily Deal today is Beat Hazard at 75% off, for $2.50.
Direct2Drive.com has a sale for Macintosh gamers; the Brothers In Arms Double Time Pack which contains the first two games in that series for $19.95. Again, this is Mac only.
GamersGate.com still has your best chance of finding a good deal at the moment. They're in the second week of a month-long Summer Sale that has close to 400 games on sale, some of them at 90% off. There are some blockbusters like Dragon Age 2 marked down by half, and plenty of indie games at two dollars or less. This cornucopia of fun is our BEST DEAL OF THE DAY.