ARKaMAN

The Source SDK is a software development kit compiled by Valve Software that is used to create maps or mods for the Source engine, with the exception of Left 4 Dead, Left 4 Dead 2, Alien Swarm, and Portal 2. These games are packaged with their own SDKs, called the Left 4 Dead Authoring Tools, the Left 4 Dead 2 Authoring Tools, the Alien Swarm SDK and the Portal 2 Authoring Tools respectively. They are all tweaked versions of the Source SDK, but tailored to suit the needs of the games and their engine builds. -wikipedia.org