After much chatter about the subject, Valve has finally unveiled the beta version of Left 4 Dead's authoring tools, that will allow you to create character skins, maps, models, props, and more.
The L4D Authoring Tools include modified versions of the same tools we used to produce Left 4 Dead.
Using the L4D Authoring Tools, you may create your own campaign maps, character skins, 3D models, sound effects, and music and load them into the game.
This release also introduces a pair of plug-ins for (Google) SketchUp, a 3D modeling program you may download for free. You may use these plug-ins to create building blocks and props that can be imported into Hammer for use in game levels.