The Left 4 Dead Authoring Tools or SDK (Stage Development Kit) is the modding tools program 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 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 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 and allow users to make their own custom maps, which are saved in .vmf format. Compiling tools are built into Hammer, making compiling maps easy, though time-consuming.
It is also possible to decompile .bsp map files back to .vmf files, that can be viewed and loaded by Hammer for educational purposes (the maps could be someone else's property, respect copyright!). The free public program BSPSource allows this operation. If the .bsp map file is inside a .vpk file you will need to open it with GCFScape, extract the .bsp file from the maps folder to any other place outside the .vpk and then it can be decompiled.
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. Latest versions of this tool also allow the users to "re-skin" models by exporting and importing .bmp files related to the model.
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.
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 who wish to have a unique special effect in their map.
The SDK also has other tools, but these can only be accessed in the bin folder of the executable directory. 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 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.
The SDK comes with sample materials to help new users. Sample materials include the "Dead Line" campaign (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.
The SDK comes with plugins for the Google program Sketchup, so those that users of Sketchup can port what they have created into Left 4 Dead with ease.