I just posted a new reverse geocoder for the US based on the Tiger/Line 2005SE database. It's not as accurate as my USPS based system but it does cover areas that mail isn't delivered like rural routes, interstates and other roads. The zip codes returned are not validated, so I don't recommend using these for anything but informational purposes at this time. There are 90,000,000 records in the database.
Parameters: (case sensitive)
The return is an xml file. This is a simple REST interface.
The usual 150 hit limit per every two hours holds on this site as well. If you need more, let me know.