The SDK (software development kit) for Left 4 Dead is released open beta.
If you own the game, the SDK mod tools are now available through Steam.
Valve has also released SketchUp Source tools, which allows modders to create content for the Source engine using Google SketchUp. Which is Free!
The list of features:
Updated Hammer, the Source level editor
Suite of command line compiling utilities, such as studiomdl and map compiling tools
Updated Particle editor
Example models: common Infected, a breakable prop, and explosives
A set of example particle effects
So update your Steam and download Left 4 Dead SDK.
I have already began to record video tutorials and they will be released soon.
Here is some of my notes that I am planning to make videos on.
Section 2 has already been finished.