Tom's Terrain Tools makes it a lot easier to bring realistic terrains and landscapes into Unity

The tools use distribution maps you have created in dedicated 3rd party tools and apply them to the terrain in Unity.
Now with incremental splatmapping, you can start from scratch or add and improve to your existing terrain.

You can import maps for texturing, trees and forests, grass and details and even create entire terrains from nothing using the AutoMagic feature.

In addition, there are a few extra scripts thrown in as well as many examples to get you started. You will receive full source code for all scripts.

More details can be found online

Forum thread:
http://forum.unity3d.com/threads/toms-terrain-tools-new-release-version-2.70480/

Support & reporting issues:
https://github.com/unitycoder/TomsTerrainToolsUnity