This is a nice little piece of coding that shows a visual representation of how zip codes break down around the US. It's interactive, so as you type out a zip code you'll get to see it drill down. Don't forget to press Z to turn the Zoom animation on.
This ties in very nicely to what I'm doing in my regular job (the address matching problem). Recently I noticed that I only get about 3 digits of signifance out of my zip code matches, so I'm curious to see just how large of a physical area such situations tend to represent.