Universcale is a neat interactive Flash website by Nikon (unveiled last year, if I'm not mistaken), which allows you to play around with the idea of "scales." With a simple click of the mouse, you can see objects of varying sizes, from femtometers (10-15) to lightyears.
Link - Thanks wade7!
http://micro.magnet.fsu.edu/primer/java/scienceopticsu/powersof10/
*requires Sun Java