I am working on vorfract, a Voronoi voxel world which uses Voronoi cells as voxels instead of the regular cubic ones.
It's an early version so many (if not most) things that would be in a game are still missing at the moment (though planned on paper).
One interesting thing is that because the Voronoi voxels are so malleable, things like round or hollow worlds are a pretty natural outcome.
In parallel to working on it, I've been mostly using it to build and improve my own voxel castle with a pointy roof, slanted walls, custom built stairs, tables, chairs and all those nice things.
In the last update I added a CRT filter and some pixelation options as I find that appealing. As for the future (maybe as the next thing?), I guess I'd probably like to add the ability to build less blocky things like round towers (or flower vases!), but that is still very much a work in progress.
The Web version (WebGL) is completely free (and will stay that way):