This one is little more than a tech-demo right now, but it’s pretty awesome for that.
It’s called Outerra (at least the engine is) and it uses fractal generation to create a world that scales from space flight all the way down to blades of grass.
You can download the tech demo and fly and drive around an Earth-like (and size!) planet.
http://www.outerra.com/
Closer to completion is the fun looking StarForge. I think of it as a cross between Starship Troopers and Minecraft with modern HD graphix. It also has the ability to play on open worlds that extend all the way from the surface into space, and mining below the ground also.
Free for now, but it’s pretty alpha quality and apparently needs a decently beefy rig until they get the performance quirks sorted.
http://www.youtube.com/watch?v=YxBSYit49c8&feature=youtu.be