From Zip Postal Codes:
The 'US Zip codes' package will provide you with the extended US Zip codes database as well as US geographical data like latitude and longitude. The 'US Zip codes' package includes the information on: US Zip code, Latitude, Longitude. 42321 precise records are in the 'US Zip codes' package. US Zip codes are in five-digit format. Latitudes and longitudes are to within 0.0001 (4 digits after the comma). The 'US Zip codes' package can be EASILY imported to ANY database. The downloaded file size is 353 KB. The uncompressed file size is 957 KB. The downloaded file is in CSV format. The 'US Zip codes' package is easily to integrate.