Today Valve announced the first downloadable add-on for Left 4 Dead 2, entitled "The Passing". It will be released in Spring 2010.

The "Passing" takes place after the "Dead Center" campaign that shipped with L4D2. The four Survivors from L4D2 meet up with the quartet from the original Left 4 Dead, and will presumably kill twice the normal amount of zombies together. The sweet cross-over adds a new co-op campaign, a co-op challenge mode, and new arenas for the Survival/Scavenge/Versus modes. A new firearm, melee weapon, and "uncommon common" zombie will also be included.

L4D DLC was free on the 360 but they had to shell out a few bucks for the second.