L4D2 mapping & add-on tools released

Additional details are available on this L4D blog post.  Here's the short version:

To play custom maps, you'll need to download the Left 4 Dead 2 Add-on Support application from your Steam client's "tools" tab.

To make custom maps, download the Left 4 Dead 2 Authoring Tools from the same location.

EDIT: Hammer still takes a long, long time to load.  So be ready for that.