Downloadable Content

Downloadable Content is content which was not released with its game initially. Said content was/is released by Valve. Thus far, there have been two downloadable content packs, one of which (The Survival Pack) was free for both PC and Xbox 360 users. The other, Crash Course, is free on PC, but costs 560 Microsoft Points to buy and download on Xbox 360.

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 against 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.

Crash Course
Valve has announced a new DLC pack titled Crash Course which includes a new campaign and more features. The campaign is touted as bridging the gap between No Mercy and Death Toll, the DLC will include all new campaign and versus maps and a more streamlined Versus experience. The poster for Crash Course has the wreckage of News Chopper 5 in the background. Other than the new campaign, the pack contains new features such as viewing teammates’ statuses and their recharge timers which was suggested a long time ago in Steam's forums and highly requested by the community. Chet from Valve noted that the game would become more balanced without having the game to change teams. Crash Course was released on September 29th, 2009.

Left 4 Dead 2
There were two rumored DLC packs for Left 4 Dead 2. "The Miracle Pack", and "The Apocalyptic Pack". Though technically they were DLCs, they were nothing but test scenarios for Microsoft. They won't be able in the game, judging by them being DLCs, meaning that they do not come with the game itself.

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."