I have been searching the internet to find a free, good quality U.S. zip code database. I have found two options that I may use:

1. GeoNames.com - This database covers all countries and has over eight million place names. It is available for download or can be accesses via a web service.
2. The TIGER database from the U.S. Census Bureau - This database covers only the United States, but has information down to the address level, so lookups can be done via address. It is available for download, or there are sites that have created web services based on the data.

Either way, I think I want to have my own database for this data, then I can do however much of whatever I want with it. I will be posting an update when I decide what I am going to use, and as I implement my choice.