https://github.com/scallyw4g/bonsai
I recently ported the terrain generators to the GPU, and increased the visible volume to 1 billion voxels cubed. I did a short YouTube video about it.
https://www.youtube.com/watch?v=bLfgjWsM1PI
I also wrote a metaprogramming language which generates a lot of the editor UI for the engine. It's a bespoke C parser that supports a small subset of C++, which is exposed to the user through a 'scripting-like' language you embed directly in your source files. I wrote it as a replacement for C++ templates and in my completely unbiased opinion it is WAY better.