Official support for community created campaigns has just been added to Left 4 Dead, thanks to the most recent update to the game. Valve has also updated the Left 4 Dead authoring tools—so get crackin' map makers!
Valve announced the long-awaited PC version update via the official Left 4 Dead blog last week, highlighting the changes that will enable support for add-on campaigns. In effect, it means more widespread access to and proper matchmaking for custom campaigns beyond the original four—No Mercy, Death Toll, Dead Air and Blood Harvest—thanks to the Left 4 Dead SDK.
Soon, we'll all be playing Dead Before Dawn—set in the Crossroads Mall from George Romero's Dawn of the Dead—and a thousand Resident Evil-inspired maps, with the obligatory nude Zoey mod installed. Hopefully a full recreation of 28 Days Later is currently brewing somewhere.