For United States locations, a close approximation of where zip codes are can be made using the file available at the census bureau in:
1999 ZIP Codes. Nice resource. As these are nearly 6 years old and supposedly the USPS updates every month... they are a bit out of date. However, for a system that is in staging phases and not ready to move to using the USPS APIs... it would be a good way to handle things. The APIs, however, allow for additional flexibility to be added.... such as address format verification, zip+4 and potentially even barcode standardization. All of these things could reduce mailing costs due to inaccurate information.