My husband wrote an app for the iPhone! It's called Zip Code Decoder, and if you buy it from the iPhone store (for a measly $1.99), it tells you what zip code you're in as well as the nearest four other zip codes to you.
This may seem like kind of a novel bit of randomness, but his idea was that it would come in handy when you wanted to use one of the many online apps that require a zip code -- ones that give you movie listings and the like -- but when you were not at home, so had no idea what your zip code is. (I know zip code is properly writtten ZIP code, but man, that bugs me.)
Plus it is kind of fun to just be riding around and to know when you travel from your zip code into your friend's zip code. But then I am easily entertained.
It apparently works for postal codes all over the world, too. Yay, Rob!