Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
MagicaVoxel - A free lightweight GPU-based voxel art editor and interactive path tracing renderer.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
Goxel - Goxel is a simple, but powerful voxel graphic editor with 24-bit color support, unlimited scene...
Sculptris - Sculptris: Enter a world of digital art without barriers.
Cubik Studio - Model in a unique cubic style. Start modelization with boxes. Move, rotate and scale cuboids.