We have a free API that includes IP to approximate area code, but not zip code, if that's of use to you.

https://www.dailycred.com/api/info.json

A few of our clients needed this, so we just made it open to everyone.

It's based on GeoIP and ua-parser.

http://dev.maxmind.com/geoip/

https://github.com/tobie/ua-parser