It's hard for us, cognitively speaking, to imagine things that are really really big or really really small, so we need things to put things in perspective.
For example: how big is a skin cell? I could tell you it's 30 micrometers, but that doesn't do you any good. In this interactive from the Genetic Science Learning Center of the University of Utah, we get perspective. The above is the start view with a coffee bean. Zoom in, and you get a much better idea of what 30 micrometers looks like.
We've seen this before with money, the Web, planets, hard drive memory, etc but the zoom for this case works really well. The scale legend in the top left is also a good touch.