Do you own both Bioshock games? If not, would you kindly head over to SteamPowered.com and check out their one-day-only sale on both Bioshock games. You can get Bioshock 1, or 2 for only $4.99. If you have three pals, and want to spread a few copies of Bioshock 2 around, you can get a fourpack of it for $14.99. This is good for today (Tuesday the 12th) only. They'll have a deal on a different game by 2K tomorrow and Thursday.
Direct2Drive.com is doing their own one-day-only sale on Dirt 2, which you can pick up for $4.95 today. D2d also does Two-Dollar-Tuesdays, and today you can get Star Defender 2, The Island Castaway, or Gish for two dollars each. Gish is actually very good, and highly recommended, especially at this price.
ImpulseDriven.com has a puzzle/ RPG called Mactabilis on sale today only for for $2.50. Meanwhile, GamersGate.com is in the middle of Space Battle Week, and you can get dozens of star-faring sci-fi games for up to 70% off. Notable among them is Star Trek Online, Digital Deluxe Edition for six dollars! That would be our BEST DEAL OF THE DAY, although you would do just fine with Gish or Bioshock as well.