The optional "Add-on Support" and "Authoring Tools" downloads are now available to owners via Steam's "Tools" tab, respectively weighing in at 350MB and 20MB. The "Add-on Support" download allows Left 4 Dead 2 to run "ANY map, including custom maps made for the original Left 4 Dead," according to Valve's Yasser Malaika.

To enable custom content creation, both "Add-on Support" and "Authoring Tools" must be downloaded, with Valve offering the following rundown of "the most useful" additions, tweaks and updates made over the original Left 4 Dead's authoring tools:

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.

We have included an updated tutorial to help you get going and as always you can join the discussion on the map designer email list. What we want for New Years is to get to play your custom maps in Left 4 Dead 2. 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.