As a Christmas present to all you Left 4 Dead 2 PC owners, Valve released the Left 4 Dead 2 Authoring Tools and Left 4 Dead 2 Add-on Support earlier today through steam. This means that you can now create custom campaigns for Left 4 Dead 2. Valve also states that the Add-on will also allow you to play custom Left 4 Dead 1 campaigns as well.

The Left 4 Dead 2 Authoring tools not only allows for custom campaigns with "A.I Director Logic", but also supports custom Scavenge mode maps as well.

For more details, please visit:
http://www.l4d.com/blog/post.php?id=3286