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

Check http://www.datasciencetoolkit.org/developerdocs#coordinates2... and http://www.datasciencetoolkit.org/developerdocs#coordinates2... You can use these APIs locally via vagrant or launch a amazon AMI http://www.datasciencetoolkit.org/developerdocs#setup

Ah, but then the data wouldn't fit into 1.6MB!

That is to say, going the other way is a bit more computationally intensive.

Interestingly enough zip codes don't cover the entire US. Not even the entire continental US. Lat/Long 40, -110 (in NE Utah) is a good test case to make sure your code handles that circumstance.

you may enjoy: http://geocoder.opencagedata.com

we aggregate OSM, Datascience tool kit, geonames, and more. All feedback welcome