A friend at work just sent around this link, and I have to say this is one of the coolest things I've seen.
Imagine a picture...now imagine every pixel in that picture being another picture. Now imagine being able to zoom in and see those smaller pictures, and as they get closer you see that they are made up of more pictures...and now prepare to spend quite a while clicking and clicking and clicking.
Simply cool. Try it yourself (NOTE: Macromedia Shockwave plugin required)