The Terrain Toolkit project was created by Sándor Moldán as part of the Unity Summer of Code 2009.
The objective of the project was to develop a toolset for the Unity Editor which would streamline and improve the workflow involved in creating terrains for games. The result is the Terrain Toolkit component - an integrated solution for terrain generation in the Unity game engine.
The Terrain Toolkit can be downloaded from the Resources section of the Unity website.
Full documentation for the Terrain Toolkit can be viewed here.