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 on Google code here: http://code.google.com/p/unityterraintoolkit/
Please note that the Terrain Toolkit is no longer downloadable from the Unity website.