Valve's gift to the Left 4 Dead 2 customization community has arrived with the release of official campaign authoring tools and add-on support. You want to play custom campaigns of your own or someone else's creation? Go nuts.
The Left 4 Dead 2 authoring tools include support for AI Director logic for variable level layouts and the option to use a single map in any mode—Versus, Co-op, Scavenge and Survival. And it looks like your original Left 4 Dead creations will work, provided those creations are recompiled in the new authoring tools.
Naturally, this is for the PC version of Left 4 Dead 2, so fire up Steam to download the add-on and creation tools.
Sorry, no Christmas break for you, modding community. But we're pretty happy that Valve got this SDK out the door a heck of a lot faster than it did for the original Left 4 Dead.
All I want for Christmas is an SDK [L4D Blog]