A russian online service Voxelart project (now closed) was created for creating from cubes. It allows to create impossible figures with some limitations. For example, it's not possible to create classic Penrose tribar with this service. The creation of figures is quite simple. You should just drag desired cube and drop it on the workspace.
Some impossible figures, which were created with Voxelart project by various visitors, are represented below. Click on image to see it in better quality.