From Game Informer:
"Valve announced today that the previously announced Left 4 Dead downloadable content for Xbox 360 and PC will be free of charge. The content, dubbed Survival Pack, will include a new multiplayer game mode called Survival and two additional campaigns (Death Toll and Dead Air) playable in Versus Mode. Survival mode is said to be similar to Horde mode from Gears of War 2; players work together to fight off as many waves of enemies as they can. The Survival Pack will be available for download this spring.
Also, a Critic’s Choice Edition of Left 4 Dead will overrun stores this spring and will include access to all the content mentioned.
Valve, you continue to serve us gamers with amazing deals. You gave us five games in one with Orange Box, and now this. Thank you."