3D planetary engine for seamless planet rendering from space down to the surface. Can use arbitrary resolution of elevation data, refining it to centimeter resolution using fractal algorithms.
Unlimited visibility, progressive download of data, procedural content generation.
Integrated vehicle and aircraft physics engines. Embedded web browser for web service integration and more.
http://www.outerra.com/Basically, a procedurally yet realistically generated earth you can explore with vehicles.
I've yet to try it, as it's downloading, but it looks pretty promising. And it supports the Oculus Rift.