It's not free but I believe that Qubicle [http://www.minddesk.com/] is the "goto" voxel editor.

I'm the coder of Smith and Winston [http://www.indiedb.com/games/smith-winston] and we've found Qubicle to be the best by far.

Adding VoxelShop to the list: https://blackflux.com/node/11