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 four 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 the Xbox 360.
 * The Passing (Left 4 Dead 2), which was free on PC, but costs 560 Microsoft Points to buy and download on the Xbox 360.
 * The Sacrifice (Left 4 Dead and Left 4 Dead 2), which was free on PC, but costs 560 Microsoft Points to buy and download on the Xbox 360 for each game.

[DLC download instructions and details are requested―some people can't find it]

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 Edition.

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 details the circumstances leading up to the events of The Passing.

The Sacrifice takes 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. The Sacrifice takes place in Rayford, Georgia, the same city The Passing takes place in.

A 190-page digital comic was released in four parts throughout September and October 2010 exploring the Survivors' backstories and how Bill died.

It was released for both Left 4 Dead and Left 4 Dead 2 on October 5, 2010. If downloaded for Left 4 Dead 2, players will still play as the original Survivors, as well as having weapons and Special Infected from Left 4 Dead 2. In addition, No Mercy was also made available for those who download it for Left 4 Dead 2 (see below).

Pre-order Baseball Bat
In a technical sense, the preorder Baseball Bat is the first DLC for Left 4 Dead 2, although it is only a single item. Players who had preordered Left 4 Dead 2 would find a Baseball Bat in every safe room. With the release of The Passing, the Baseball Bat lost this restriction and now spawns like a regular melee weapon.

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.

The Sacrifice
The Sacrifice is a port from Left 4 Dead. Players take control of the original survivors, but play with additions unique to Left 4 Dead 2 (like the new Special Infected). However, they are only selectable for both The Sacrifice and a Left 4 Dead campaign being released for Left 4 Dead 2―No Mercy.

Cold Stream
On February 16, 2011 Valve anounced a new (non-canon) DLC. This DLC is a custom campaign developed by Matthew Lourdelet, the developer of 2 Evil Eyes.

Even though this is not a story-canon campaign, it will still be released to the Xbox 360 as well as PC and Mac.

This DLC will also feature the three original campaigns for Left 4 Dead (as well as Crash Course) which is not yet released (Death Toll, Dead Air, Blood Harvest).