Quad Software GROME 3.11.06 Win64
Grome is an environmental modeling package developed by Quad Software dedicated for procedural and manual generation of large virtual outdoor worlds suitable for games and other 3D real-time simulation applications.
This is a great introduction to Grome modeling that also deals practical editing details and with integration of the scene data into well known engines: Unity3D, Unreal (UDK) and Ogre3D.