L4D2 Authoring Tools have been released. Start up your Steam and download the L4D2 Authoring Tools and the L4D2 Add-On.
Merry Christmas from Valve.
L4D2 Add-On Support allows the ability to play custom campaigns all you need to do is install the add-on support package. Open up the Tools tab. Look for L4D2 - Add-on Support, click and install. After installation, you do not need to run this application, it will be active.
Downloading L4D2 - Authoring Tools will add applications, utilities, and example files that are useful for building L4D2 game add-ons.
Quote from L4DBlog
"Gamemode Logic: Designers can now make a single map that can behave differently depending on the gamemode in which it is being played. This can dramatically reduce the file size of Campaign add-ons, as the same map can be used for Co-op, Versus, Scavenge, and Survival.
Director Logic: Maps can now alter their layout and behavior depending on how "angry" the A.I. director is. The better the Survivors are doing, the angrier the AI Director seems to get. This allows you to base variable paths and other events on the player's skill and current conditions.
Scavenge Mode Support! All the elements map authors need to support Scavenge in their maps. This includes game props as well as a full example in the form of Deadline 2, an updated tutorial campaign.
For the best user experience and to take advantage of the new map and nav mesh formats, Left 4 Dead 1 Map Makers will want to compile Left 4 Dead 2 specific versions of their map in the new tool."