VoxelFarm Realtime is Minecraft in a gorgeously realistic procedurally generated world
Miguel Cepero's Procedural World project was already an insanely impressive example of the collision between maths, nature and beauty; creating vast landscapes and detailed structures. Recently his work has taken a familiar, if equally spectacular turn, adding a Minecraft style WYSIWYG landscape editor and building system that lets him shape his random worlds. There is a video. Try not to gawp.
If that wasn't enough, the engine's building tool can go further and shape the blocks, adding curves and smoothing. As Cepero explains in this blog post, "you still lay boxes the same as in Minecraft, but then you can go back and alter them. I saw that a single operation was enough to produce both curved and straight angled surfaces. If you applied it gently you would get curves. If you applied it more, it would straighten out." There's another video showing this system at work.
The engine, codenamed VoxelFarm Realtime, is still firmly in the development stage. It's not yet known whether this is building towards a commercial release, but you can keep up with the project's progress over at the Procedural World blog.