Custom Campaign Support for Left 4 Dead 2 Official

Valve has released the Left 4 Dead 2 Authoring Tools and Left 4 Dead 2 Add-on support as a Christmas present to the L4D 2 community. The new content allows players and designers the necessary tools to build their own maps or play maps designed by others.
According to the blog post, their aren’t any direct links available to this package in the game’s UI so follow the instructions below to proceed.

PLAYERS: If you are not looking to design but play custom campaigns all you need to do is install the add-on support package. Just go into steam and open up the Tools tab. Look for the entry labeled Left 4 Dead 2 – Add-on Support click and install it. This download is sizable, but will update your L4D2 installation to allow running ANY map, including custom maps made for the original Left 4 Dead. Once installed, you do not need to run this application, it will always be active.

DESIGNERS: Besides also downloading the above file you will need to download the Left 4 Dead 2 – Authoring Tools. Downloading it will add applications, utilities, and example files that are useful for building L4D2 game add-ons. These are similar to the tools released for Left 4 Dead, but include many updates and new features.
Among the most useful are:
  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.

Valve has also updated tutorials to help you get going and as always you can join the discussion on the map designer email list for more info.