I've never played with any geolocation tools before but I thought I'd take a whack at it. After stumbling across GeoHelper, I thought I'd test things out with it. Well, after finding that the only reverse postal code lookup is Canada Post, I might as well make my own.
Download GeoHelper and script at the bottom of the page and place it in the same directory. That's pretty much it, the code explains itself. It can also do a reverse lookup on IPs.