Downloadable Content

Survival Pack
Released on April 21st 2009, the Survival Pack was confirmed by Valve on February 5th 2009. The Survival Pack is completely free and is available through Steam and Xbox LIVE.

First off, you can now play the two campaigns Death Toll and Dead Air online in Versus mode. These maps have been tweaked slightly to remove some closets and to balance gameplay. For example, downstairs in the Boathouse Finale has been significantly emptied to prevent corner camping, and more walls break open.

The anticipated Survival Mode is released which pits the The Survivors againsts endless waves of the horde, as well as multiple Special Infected. Players can hold out in 16 maps, 15 of which are extracts from Crescendo Events and Finales from the four original campaigns. These maps have also been tweaked to remove exploits and to make the whole experience much harder. The last map is in the new campaign: The Last Stand, and currently has one map: The Lighthouse.

Seven new achievements have been released for the PC version of the game. There are no new achievements for the Xbox 360 version.

Unnamed DLC
Valve has announced a new DLC pack. Absolutely nothing is known about this DLC at the moment, but Valve has said that "Details will be revealed shortly".

Trivia

 * The Last Stand could be a nod to a well known flash game of the same name, created by Armor Games, which also has zombies and "survivors"