Some time ago I created a voxel editor, just for fun (even though I might use in future projects).
To use, import a image (16x16 pixels) and use it as base to add or remove voxels.
It saves the file as a .vxl format, if you want to use it for games or something, the format is quite simple, just a raw 16x16x16 array of RGBA colors (4bytes).
Download it here: www.pascalgameengine.com/downloads/voxeled_alpha.rar
I'm open to any suggestions