Slab6 is described as 'voxel editor that Edits, VOX, KV6, and KVX files' and is an app. There are more than 10 alternatives to Slab6 for a variety of platforms, including Windows, Mac, Linux, Online / Web-based and Steam. The best alternative is 3D-Coat. It's not free, so if you're looking for a free alternative, you could try MagicaVoxel or Goxel. Other great apps like Slab6 are VoxelShop, Voxel Builder, Avoyd and Acropora.
Voxel Builder lets you easily create and edit 3D voxel graphics right in your browser. Simply select a colored block and click to draw. Zoom in and out with the mouse and click and drag to navigate around the 3D canvas.
Model in a unique cubic style. Start modelization with boxes. Move, rotate and scale cuboids. Apply one or more textures per cube, disable any face, edit UV coordinates and create any character parts without the constraints of voxels.