Visual tools for understanding the scale of the world are always interesting to explore. Powers of 10 examples like this one on the Florida State University site are a great example.
Universcale is another, produced by Nikon. It uses a Flash animation along a scale line you can use to browse up and down. Be sure to mouse over the black blobs to find out what they are. While not as graphically rich as the first, it goes to the extremes of known science.