Ready Reference

U.S. Postal Service & Zip Code

Addressing Standards for Puerto Rico and the Virgin Islands
http://www.usps.com/ncsc/addressstds/addressstdsmenu.htm
General information about formatting Puerto Rican and Virgin Island addresses, residential and apartment address formats for Puerto Rican addresses, components of Puerto Rico delivery address lines, commonly used descriptive words and abbreviations that may appear in Puerto Rican addresses.

Area Code and Zip Code Look-ups
http://www.melissadata.com/keywordsearch/zipcode.htm
Free zip code and address information.

Official Postal Service Abbreviation
http://www.usps.com/ncsc/lookups/abbrev.html
United States postal abbreviations for states, military, commonwealths, and territories.

US Postal Service
http://www.usps.gov/
Lookup ZIP Codes, Post Office locations within a close proximity to a specific address, postage rates.

ZIP Code Lookup and Address Information
http://www.usps.com/ncsc/ziplookup/lookupmenu.htm
US Zip Code information from the proverbial horse's mouth (the US Postal Sevice). You can look up the Zip+4 code for any address, or enter a city and state to see all Zip Codes associated with that city.