Ben Fry is the author of Visualizing Data, and describes the ZipCode project in his book. Each dot on the map is one zip code, and as you type a zip code, it highlights all of the dots that share that portion of the zip code. You can turn on the "zoom" feature that zooms farther into the map for each digit you add.
This is built with the open source Processing tool that was recently released to the world as version 1.0.
Here are all of the zip codes that start with "6"
Thanks Steve for sending the link!
Here's a link to Ben's book on Amazon: