So tonight I discovered this thing:
http://www.stonequest.de/

And it looks completely awesome.

Of course it falls in the same sandbox realm as Minecraft, but it looks beautiful and it's not limited to such a hard grid as Minecraft and so many other voxel sandbox games are.

Also, check out his video on non-repeating textures:
https://www.youtube.com/watch?v=jUoQIZ0Rkx8

Super cool!