Goxel
Open Source Voxel Editor for Windows, Mac, Linux, iOS, and Android.
By restricting the volume along a 3D grid, just like pixels do in two dimensions, voxels make 3D editing as intuitive as drawing in 2D.
Voxel art is used in many video games, and also by artists as a standalone style.
Get the desktop version now for free in the download page, or try the webgl live version directly in your browser.