Mailing Address Verification, Etc. Get County from Zip

Question

It seems fairly common today for websites to perform some address verification behind the scenes. For example, the 4 digit extension on zip codes is commonly filled in. In addition to filling in the 4 digit zip code extension, I would like to know what county the address belongs to.

This will be utilized in an ASP.NET 3.5 application that already has all the hooks in place for ASP.NET AJAX along with jQuery.

What services out there would allow for this to be done? Web services, 3rd party includes, etc.

UI Structure

ADDRESS_LINE1
ADDRESS_LINE2
CITY
STATE 
ZIP

Answer 1

The US Postal Service's website lists many address verification services.Publication 28 (PDF) is the addressing standard for US addresses. Unfortunately, it's more complicated than your schema.

Answer 2

Depending on the license option you need UPS can be free.

Answer 3

If you've got cash, NetAddress is built for .NET.

It's also been asked on SO before: here.