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.
Cubik Studio - Model in a unique cubic style. Start modelization with boxes. Move, rotate and scale cuboids.
Sculptris - Sculptris: Enter a world of digital art without barriers.
SpriteStack - 3D pixelart editor based on sprite stacking technique.
meshmixer - meshmixer is an experimental 3D modeling tool for making 3D mashups without too much hassle.