MagicaVoxel - A free lightweight GPU-based voxel art editor and interactive path tracing renderer.
PixBuilder - Free Photo Editing Software for PC
Goxel - Goxel is a simple, but powerful voxel graphic editor with 24-bit color support, unlimited scene...
PhotoDemon - Lightweight portable BSD licensed photo editor for Windows focused on performance and usability.
Cubik Studio - Model in a unique cubic style. Start modelization with boxes. Move, rotate and scale cuboids.
Fotografix - In addition to being totally portable, Fotografix has a ridiculously small footprint.