Could someone point me best place to get the most valid US ZIP Codes database? We are planning to integrate into our User registration.
Zipinfo.com sell a straight list showing city / state / zipcode for a one-off US$20. There might be similar products available for freee, but I've never come across any.
There are also websites offering individual look-ups for free, but I guess that won't be useful for you, as you will be wanting to automate the process.
Hope this helps.