Author and produce your very own vision of a Zombie Apocalypse! The beta for Valves Left 4 Dead Authoring Tools is now publicly available. Create your own campaign maps, character skins, 3D models, sound effects, and music and load them into the game. Let your nightmares begin.
Source: L4D Blog
If you want to get started on the beta, we suggest joining two mailing lists:
And the general Source mapper’s list. http://list.valvesoftware.com/mailman/listinfo/hlmappers
This is beta software, so if you have any problems or issues, please send them to the l4dmapper email list.
We have also updated our developer Wiki http://developer.valvesoftware.com/wiki/Authoring_Tools/SDK_(Left_4_Dead)