Left 4 Dead SDK Beta Released
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.
The only mod you need is replacing the tank music with Ride of the Valkyries!
Nuclear engineers build weapons. Civil engineers build targets.