Counter-Strike: Global Offensive will be available on August 21, assuming it doesn't fall victim to Valve Time, of course. The game will be available on PlayStation Network, Xbox Live, and Steam for PC and Mac, simultaneously. It will be available for "approximately $15" on all platforms.
CS: GO includes updated versions of classic CS content, in addition to new gameplay modes, new maps, characters, and weapons. This game is supposedly Valve's only content for E3 this year, much to the dismay of Half-Life fans.