The Federal Communications Commission (FCC) is an independent agency of the United States Government created to regulate interstate communications by radio, television, wire, satellite, and cable. Amateur licenses and call signs are issued by the FCC for 10 years upon successfully passing an amateur radio examination. After 10 years, or upon moving to a new location, a ham must update his license. The FCC maintains the Universal Licensing System and every entity holding a radio license in the US is issued a ULS number. Any call sign may be researched by searching the FCC ULS. A link to the FCC is below. A second link is provided which is to the FCC via the Amaerican Radio RElay League and can be used to find all amateurs within a zip cope or city.
Link to main FCC databace lookup:
http://wireless2.fcc.gov/UlsApp/UlsSearch/searchLicense.jsp
Link to ARRL FCC database lookup: