The non-profit I work for has an automated registration system that matches a potential volunteer to a clinic near them. In order to do this, you need a zip code database with lat/long entries, and a script to run the great circle formula to determine the distance between two zipcodes. I thought I would post the information I have, in case it is useful to anyone else.
Free zip code database from CivicSpace
Last updated August 2004, ~32000 records
$5 zip code database from Team Redline
Last updated June 2006 (monthly), ~42000 records
Great circle and two approximation formulas
I really can't vouch for the accuracy of the zip code db's, but they both say they came from USPS sources.
Zip code database
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