MI-80

Valve, creators of best-selling game franchises (such as Half-Life, Portal, Team Fortress, and Counter-Strike) and leading technologies (such as Steam and Source), today announced the first game add-on for its co-operative zombie thriller, Left 4 Dead 2 (L4D2), is in development and targeted for release in early 2010.

http://store.steampowered.com/news/3237/