I live in San Francisco, so my Zip code (postal code) is 94107. But have you ever wondered where the 5 digits in your zip code come from? It turns out it's based on a hierarchical system: the initial 9 represents the western region of the US; the 5 represents the San Francisco area within that, and so on. You can see the system in action in the elegant zipdecode data visualization by Ben Fry (also known as the author of the Processing data visualization language):
Once the visualization has loaded, type your zip code. The key though is to do it slowly, digit by digit, so you can watch as it zipdecoder zooms into the levels of the hierarchy.
Ben Fry: zipdecode (with thanks to reader KW for the suggestion)