It's a Flash applet to help visualize the relative scales of things, from the Planck length all the way up to the universe itself.
(Being an amateur astronomer I have to nit-pick something, though. The size he states for the size of the universe is wrong. The observable universe is 13.7 billion light-years old and 93 billion LYs across. The full universe is much larger than that. The app states 13.7 BLY across for the observable universe. It's still a great piece of work, though.)