Like are you talking minecraft style? Or just a useful tool for terrain creation?
If it's the latter, I'm a fan of world machine. It's flexible, versatile, and can export to just about any SDK you can think of. The program was created by one guy, and its interface is flowgraph based. You can get a trial version at http://www.world-machine.com/
Its got alot of cool options that save you some of the labors usually reserved for environment artists, here's an example of something I exported form world machine to Cryengine 3
Hope that helps! This was my first post, hello world!