Authoring Tools/SDK (Portal 2)
The authoring tools is a special edition by Valve released as an update to the original Source SDK. Currently, the program is in an Open Beta state, which allows owners of Portal 2 to create and run their own custom-made maps. However, there are possible bugs within the system, so use of the program is cautioned.
To download the program, go to Your Library in the Steam UI -> Tools -> "Portal 2 Authoring Tools - Beta". Note that most situations in Portal still apply to the original Portal's level development system (which can be found in the Portal Level Creation category). However, special units (like the Aerial Faith Plates) will be placed in this category.