The other day I stumbled on a really cool website, where they offer a free API to calculate distance between two US ZIP codes, all ZIP codes in a specific radius from a given ZIP code, as well as a few other ZIP code related functions.
The results can be returned in different formats, like JSON, XML and (in some cases) CSV. Highly recommended! The URL is http://zipcodedistanceapi.redline13.com/
I am considering creating a Lotusscript class for some of these functions, so stay tuned!