usps.gov

["Aaron Swartz" <xx@aaronsw.com>]

So one of the things we want to do on Watchdog is let people enter
their zip code and find out information about their neighborhood. But,
outrageously, the USPS's database about which zip codes are where
isn't available publicly. It's another one of these absurd government
mail-away-for-the-DVD things. (Did they all get some memo on this or
something?) Anyway, it looks like a couple thousand dollars for the
important databases. You interested? I can PayPal the money if
necessary.

http://www.usps.com/ncsc/
http://www.usps.com/ncsc/addressinfo/addressinfomenu.htm
Manual on ordering: http://ribbs.usps.gov/files/Addressing/PUBS/AIS.pdf

The important things seem to be:

 - Five-Digit ZIP ($500; lists the streets for each zip code)
 - ZIP+4 ($900; lists the addresses for each ZIP+4 code)
 - City State (free with the above; maps to city and state names)
 - TIGER/ZIP+4 ($700; lets you map the zip codes with Census data)

Delivery Statistics ($350; lists the number of deliveries to each zip
code) also looks kind of interesting.