Left 4 Dead's Survival Pack DLC will be released on April 21 on the Xbox 360. The Survival Pack will be free to download, and adds the new Survival multiplayer game mode, where up to four players are given time to prepare before taking on hordes of rampaging undead as long as possible.
Not only is there a new mode for the game, the content pack will add 12 new maps -- well, kind of new, anyway. 11 are sections of existing levels and one ("Lighthouse") created especially for Survival Mode. Survival Pack also adds versus mode to Dead Air and Death Toll, the final two co-op campaigns from the original game release.
The Left 4 Dead: Critics Choice Edition that contains the original game plus the Survival Pack comes out on April 21st as well, in case you're late to the party. No word yet on whether the PC release of this extra content will come on the same day.