UnrealEd!

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, and Alien Swarm. These games are packaged with their own SDKs, called the Left 4 Dead Authoring Tools, the Left 4 Dead 2 Authoring Tools, and the Alien Swarm SDK respectively. They are all extremely similar to the Source SDK, but tailored slightly to suit each game's needs.