Apple's App Store, the PlayStation Store and its own PC game store. You can find Prince of Persia Retro, Silent Hunter Mobile and a few other iOS titles on sale in the App Store for just a buck. Ubisoft's game packs on PSN are at half price (one features the old school Prince of Persia bundled with Cell Factor, and the other contains Scott Pilgrim, TMNT Reshelled and Voodoo Dice) and the official online store for Ubisoft has 33% off most of its PC titles (with a full list after the break).
None of these sales are super crazy (though that 99 cent deal for the original Prince of Persia on iOS is pretty good), but if you've been thinking of picking up something from the Assassin's Creed series or a Tom Clancy title, this deal should get you at least a few bucks off. The sale lasts until midnight PST on Christmas Day.