There are some yummy deals online, but you'll have to act fast, as most of these are only up for one day. Steam is in the third day of their 2K Week, and today only (Wednesday the 13th), you can get Borderlands and any of it's DLC for 75% off. Even if you already have the game, this is still a great chance to get all of the DLC packs for just $2.49 each. Or you can get the Game of The Year Edition for $7.50, which has all of the DLC anyway.
Elsewhere, Direct2Drive.com is also doing a Daily Deal which will let you snag the indie Diablo-esque Action RPG Torchlight for $3.75. ImpulseDriven.com has a one-day-only sale on the puzzle game The Machine; you can get it for $3.75 too.
Amazon.com has deals on downloadable games through Easter. They're marked down by as much as 75% and while many of them are mediocre casual games, you can find some hardcore titles hidden amidst the avalanche of Diner Dash clones and "Hidden Object" hunts. Notable are Sniper: Ghost Warrior for $10.98, and Gray Matter for 19.48 (A personal recommendation of mine).
GreenmanGaming.com has several small price breaks at the moment, but an eye catcher is the 29% off Rift, which you can get for $34.99. They're also selling the Digital Collectors Edition of Rift with a smaller discount, at $49.99.
While several of these sales are terrific, we're giving Steam our BEST DEAL OF THE DAY award for the GOTY Edition of Borderlands for $7.50.