Many websites allow you to conduct zip code searches and reverse zip code searches and lookups for free.  Following is a list of zip code search sites:

Addresses.com	Includes People Search, White Pages, Yellow Pages, Public Records, Criminal Records, Reverse Directory, ZIP and Area Code search tools, Phone Book Search, and more.
Cedar Zip Code Search	Zip Code Search
Jay Computer Services	Zip code Area Code Lookup
National Address Server	Returns Zip Code+4
U.S. Post Office City State / ZIP Code Associations	Enter the city get all zip codes for that city or enter the zip code and get the city that it belongs to
U.S. Census Zip Code List	Zip code text file containing a list of all zip codes
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Zip Code Books	Zip Code books and zip code directories from Amazon.com
ZIP+4 Code Lookup	US Post Office Zip+4 Code Lookup tool
Zip Code Download	US postal service zip code database
Zip Code Lookups	Excellent zip code search resource allows you to: Look up ZIP Codes and city names nationwide Displays ZIP Code boundary maps and aerial views of five-digit ZIP Codes Display a list of ZIP Codes within a city Display a list of ZIP Codes within a county Display a list of postal Carrier Routes in a ZIP Code Display a list of street names within a ZIP Code Display a list of ZIP Codes in a radius from a center ZIP Code Display a list of house numbers within a ZIP Codes Look up and verify U.S. addresses Look up and verify Canadian addresses Locate anyone nationwide. Search by name, phone number or address
Zip Code Lookup Tool	Free ZIP code Lookup for any valid ZIP code, find the city and state, plus: county name and FIPS code, area code, time zone latitude and longitude, MSA/PMSA
Zip Search	Search by zip code, or by city and state