Downloadable Content

Downloadable Content, also known as DLC, is content which was not released with its game initially. Said content was/is released by Valve. Thus far, there have been three downloadable content packs:
 * The Survival Pack (Left 4 Dead), which was free for both PC and Xbox 360 users
 * Crash Course (Left 4 Dead), which was free on PC, but costs 560 Microsoft Points to buy and download on Xbox 360
 * The Passing (Left 4 Dead 2), which was 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. It is preloaded on the Game Of The Year Editon.

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 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
A few months after the announcement of Left 4 Dead 2, Valve announced a new DLC pack titled Crash Course for Left 4 Dead which includes a new campaign and more features. The campaign is touted as bridging the gap between No Mercy and Death Toll, the DLC includes a new campaign consisting of two maps and about 30 minutes of gameplay as well as versus maps and a more streamlined Versus experience. The poster for Crash Course has the wreckage of News Chopper 5 in the background.

The pack also makes it possible to view 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.

The Sacrifice
This DLC will detail the circumstances leading up to the events of The Passing.

The Sacrifice will take place after Blood Harvest when the Survivors are rescued by the military. While Bill dies in canon, players can choose who to sacrifice in the DLC. Based on the quotes of the original Survivors (and Bill's body), the DLC may or may not take place in Rayford, the same town The Passing is in.

A 150+ page digital comic is set to be released in 4 parts throughout September 2010 exploring the Survivors' backstories and how Bill died.

It is set to be released for both Left 4 Dead and Left 4 Dead 2. No Mercy is also available for those who download the DLC for Left 4 Dead 2, Even though youll play the Sacrifice and No Mercy as the original survivors, you still have weapons and special infected from Left 4 Dead 2

The Passing
On December 14th, 2009, Valve announced the first DLC for Left 4 Dead 2. Entitled "The Passing", players will control the new Survivors as they meet up with three of the original Survivors from Left 4 Dead in a campaign that takes place between Dead Center and Dark Carnival. A new firearm, the M60, and a new melee weapon, the Golf Club, appeared, along with a new "Uncommon Infected", the Fallen Survivor. New Survival, Versus, and Scavenge maps were made available. It was released April 22, 2010 for free on the PC and for 560 Microsoft Points on Xbox 360.

In a technical sense, the pre-order Baseball Bat is the first DLC for Left 4 Dead 2, although it is only a single item.