For simply ZIP code look-up here are some other examples:
- I think the exact url for USPS is http://www.usps.comzip4
MIT Visual ZipCode Decreaser is a Java applet which maps United States zip codes on a map, reducing possible matches as the number is typed
However after reading through all the posts I realized that you are looking for integration into your website.
Look at the host it yourself solution. I believe from your post that this may be what you are looking for. They have several solutions which inexpensive.
I hope this helps...