I got this product for free from 3M.  It comes with a lot of pieces so you can hang more than a couple of frames.  These are a great alternative if you don't want to ruin your walls with nails.  However, you can really only use these hooks for frames that have a wire since the hooks have a very thin opening and the wire can't be too close to the top of the frame, otherwise the white backing of the hook will show.