3D-Coat, 3D Pixel Artist, VoxelShop, VoxEdit, MagicaVoxel, Kv6ToSchematic, Acropora, Qubicle Voxel Editor, 3D Slash, VoxelMaker, Voxel Builder, Goxel, Cubik Studio, Voxelator, Project Vox, SpritePile, Curious Blocks, Slab6 are some alternatives to SpriteStack.