Zip+4 Code Look-Up
The U.S. Postal Service's look-up site that allows you to search for zip codes by delivery address.
http://zip4.usps.com/zip4/welcome.jsp

City/State/Zip Code Associations
Find a city by entering a zip code or a zip code by entering a city.
http://zip4.usps.com/zip4/citytown_zip.jsp

National Address Server
"Given a valid U.S. postal address, this server attempts to rewrite the address in the proper format along with the ZIP+4 code. If it is successful, you can retrieve a Postscript or a GIF file of the address for printing, with a barcode! You can also view a street map of the address, from 2 different Internet map sites (MapBlast and MapQuest)."  --info taken from National Address Server website.
http://www.cedar.buffalo.edu/adserv.html

International Postal Codes
Links to postal codes by country from Freesearching.com
http://www.freesearching.com/zip_codes_intl.htm