Left 4 Dead Authoring Tools

The Left 4 Dead Authoring Tools (or SDK) is the modding tools for Left 4 Dead. It is free for anyone that has the PC version of Left 4 Dead. It can be accessed by clicking on the "Tools" tab in Steam, then clicking on "Left 4 Dead Authoring Tools".

The SDK contains multiple tools which are useful for those that wish to create new material (such as levels and models) for Left 4 Dead, while more casual users can use it to see how how the content in Left 4 Dead was made.

A beta of the SDK was released on May 15th, while the official version was released on June 25th.

Hammer
Hammer is the official level editor used by Valve, which has been used on all Valve products dating from the original Half-Life. It has access to all of Left 4 Dead's textures and models and can accept custom textures and models as well. Hammer can read uncompiled maps, which are in .vmf format. Compiling tools are built into Hammer, making compiling maps easy, though time consuming.

Model Viewer
The Half-Life Model Viewer is a tool that allows the user to browse through the various models in Left 4 Dead. This tool can also play animations, making it useful for debugging new animations added by modders. For those that have no interest in debugging or adding new models, the viewer can be used to observe animations and models without getting mauled by hordes of Infected in the game.

Face Poser
Face Poser is used to create facial expressions for all characters that have movable faces, such as the Survivors and Common Infected. Such facial expressions are built from sound files, usually consisting of only speech. Advanced users can use the tool to create animations for movies using the Left 4 Dead characters.

Particle Editor
The Particle Editor is a tool that opens up Left 4 Dead and allows the user to create advanced effects while in the game, such as post-processing effects. This is mostly used by custom level editors that wish to have a unique special effect in their map.

Other Tools
The SDK also has other tools, but these can only be accessed in the /left 4 dead/bin folder. Some of these tools include vpk.exe, which allows level designers to pack their levels into a .vpk file, studiomdl.exe, a tool used to compile new models for L4D, and vtex.exe, which is used to convert images into textures usable in Left 4 Dead. More information about the tools can be found here.

Sample Materials
The SDK comes with sample materials to help new users. Sample materials include the "Dead Line" campiaign (as a tutorial for making custom campaigns), uncompiled versions of all the co-op maps in Left 4 Dead and uncompiled versions of the Common Infected, breakable woodrail prop, Gas can and Propane tank models.

Plugins
The SDK comes with plugins for the Google program Sketchup, so those that use Sketchup can port what they've created into Left 4 Dead with ease.

Left 4 Dead 2
According to Valve, the Left 4 Dead 2 version of the SDK will become available as soon as the game is released.