There are 2 repositories under voxel-editor topic.
free and open source voxel art tools - editor, thumbnailer and format converter
A highly flexible, voxel-powered game engine with an embedded editor. Create games using Lua, the native C/C++ API, and assets authored in voxel editors like MagicaVoxel or Avoyd.
Intuitive open source voxel editor; for both small and big ideas!
A curated list of awesome opensource voxel tools, libraries, games, and other cool stuffs
Particubes is a platform dedicated to voxel games, games made out of little cubes.
Minecraft Voxel-Builder for Structures and other stuff without the need to create a new world for every little structure :)