US Zip Code overlay on Google Maps

Today I had a need to find out a set of zip codes that would match a delivery area for one of my customers. I was wondering how could I find out all the zip codes over a given area...

Searching a little bit I found a website named zipmap which actually does just that. The entire US is covered and you can navigate it just like you navigate a regular Google Map view. Very practical!

You can easily move around, zoom in and out, and you even see the map with street names in the background.