I heard about this on Leo Laporte's podcast a while back, but only got around to checking it out this evening.
The Universcale website, kindly hosted by Nikon, allows you to experience the various scales of nature, from far off galaxies, right down to a single Hydrogen atom and beyond.
The interface is extremely well done.
A similar, but less visually appealing, site is Powers of Ten. Still worth a look.