Check out the
zipdecoder . It's a map of the US with a dot for each Post Office. Type in a zip code and the dots go out as they are eliminated from possibility. When the entire code is in, it tells you what the name of the Post Office you put in the code for. Show how the Zip system is laid out.
While you're at it, this guys sight has a bunch of cool Java Applets, especially if you're stoned.
