mcavic - 5:21 am on Apr 16, 2010 (gmt 0)

USPS does offer their raw database on CD at a cost. I think $1,000 a year, but I'm not sure about that. You'd need to be able to convert it to SQL or something, and if you're good with databases it's fun and powerful to work with. It will let you convert zip codes to cities and vice-versa, and validate complete addresses down to the apartment number and Zip+4.