Jeremy Lee writes "There are some 'educational' apps that transcend the merely interesting, and expand you mind with the force of a crowbar in the cerebellum. Celestia is one. Atom in a box is another. I keep it handy on my new iBook to blow the minds of chemist friends. It only runs on PowerPC Macs, but it's almost a justification for getting one. It should be used in schools to teach chemistry."
Celestia is also great (and available for other platforms) ... I couldn't download it from the main link on the page, but was able to get it from SF.net
. But if you really want to amaze your friends, turn your Mac into a Desktop Cray