Google just released the incredibly detailed Google Body Browser. It could be a valuable educational tool and it's only a little bit creepy.
It only works with browsers that have WebGL graphics support (that's just Chrome Beta for now) but you can see it in action in the video below.
[youtube]http://www.youtube.com/watch?v=KidJ-2H0nyY
One can imagine this being an incredible educational tool (and it's only a little creepy).