Valve has announced the release of the Left 4 Dead 2 SDK and Left 4 Dead 2 Add-On Support on their official Left 4 Dead blog. If you own a copy of L4D2, just go to the Tools tab of your expanded Steam window and you should see them available for download.
The Left 4 Dead 2 SDK is self-explanatory: it allows aspiring modders and map-makers to create their own custom maps, campaigns, and modifications.
What’s new is the Left 4 Dead 2 Add-On Support. According to Valve, installing the Add-On Support is mandatory for players who wish to try out L4D2 custom campaigns and mods. Custom maps from the original Left 4 Dead can also be played with the Add-On Support.
The download for the Add-On Support is about 350MB, and all L4D2 players are strongly encouraged by Valve to have it installed.