Valve have announced new downloadable content for Left 4 Dead 2 on the official L4D blog, but unlike previous expansions this one originates in the L4D community. Modder Matthew Lourdelet (2 Evil Eyes) has been working on a mod called Cold Stream, and impressed by his work Valve decided to help out and make it an official DLC but not in the traditional sense. Instead they are asking players to help out with feedback and suggestions and the community will be involved in shaping Cold Stream.
No date has been set for Cold Stream, but it will be released after Valve's Portal 2 on April 22.