I've seen (on home improvement shows) shelves that hang with hidden brackets (ex: Mount a 2x4 onto a wall and slip the shelf over it and secure from top (obviously, the shelf is about 4" thick). I've also seen (again, on TV) shelves that look like you screw supports into a wall (short tubes?) and slide the shelf over them and secure.
Where can I find something like this or do I need to build my own?