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 $6.99 USD to buy and download on the Xbox 360.
- The Passing (Left 4 Dead 2), which was free on PC, but costs $6.99 USD 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 $6.99 USD to buy and download on the Xbox 360 for each game.
- Cold Stream (Left 4 Dead 2), which was free on PC, but costs $6.99 USD to buy and download on the Xbox 360.
Survival Pack Edit
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.
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.
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. For the latter version, see below.
Pre-order Baseball BatEditIn 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. 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.
Initially, it was to be released for the first game alone, but since players would have to own Left 4 Dead to experience the events that explain The Passing, the developers opted to release it for both games. No Mercy was brought over as well because it was the most popular campaign.Both campaigns are available in every game mode. Players will still control the original Survivors, but will find items and Special Infected from Left 4 Dead 2. Five achievements were added, as in Left 4 Dead.
Valve announced a new (non-canon) DLC on February 16, 2011 . This DLC is a custom campaign developed by Matthew Lourdelet, the developer of 2 Evil Eyes and eventually made official by Valve. Even though this is not a story-canon campaign, it was released on July 24th, 2012 for PC and Mac, and August 3rd for the Xbox 360.
This DLC features the rest of the original campaigns from Left 4 Dead, Death Toll, Dead Air, Blood Harvest, as well as the later addition of Crash Course. They were ported over to Left 4 Dead 2 in a similar fashion with No Mercy when The Sacrifice was released, featuring the Left 4 Dead Survivors.
- The Last Stand and Cold Stream are the only campaigns that aren't considered canon in the Left 4 Dead Storyline.
- The Passing is, according to Valve, the most important campaign yet, due to the original and new Survivors meeting up.
- If Survival, Versus or Realism Versus is played at The Port in The Passing, Francis, Zoey, and Louis will not be at the stand where they normally appear. This is to make it fair for the Infected team as they'd never be able to reach and attack the original Survivors.
- In the Left 4 Dead 2 demo, there were two un-released "DLCs" called the Miracle Pack and the Apocalyptic Pack which would only appear after failing to load a map that the host selected and people expected these "hidden and upcoming" DLCs to be released some time later. Chet Faliszek then explained that these were basically test code.
- Mutations are included with the Passing DLC on consoles and Cold Stream on PC/Mac devices.
- ↑ http://www.shacknews.com/onearticle.x/59832
- ↑ http://www.vg247.com/2009/08/05/left-4-dead-chet-faliszek-talks-about-crash-course-dlc/
- ↑ http://www.pcgamer.com/2010/08/21/interview-valve-on-their-new-left-4-dead-dlc/
- ↑ http://www.l4d.com/blog/post.php?id=3729
- ↑ Interview: Valve reveal their new Left 4 Dead DLC