Valve has released a beta version of their authoring tools for Left 4 Dead
. The tools will allow you to "create your own campaign maps, character skins, 3D models, sound effects, and music and load them into the game." The kit includes a level editor and command-line compiling utilities, as well as example maps, props, infected, and explosives. It also brings plugins for a 3D modeling program called SketchUp. Valve has updated their development wiki
to go along with the release.