Updated with new Steam details.
PC gamers will find deals on lots of titles today. It's a great opportunity to pick up a game or two you've been meaning to play.
Two notes: Dishonored. Every retailer has it for a discounted $29.99. Batman: Arkham City Game of the Year Edition. Amazon and Steam both have it for $7.49. I recommend both titles, especially at those prices (just beware of Batman's always-odious Games for Windows Live integration).