Want to auto populate city and state from an entered U.S. zip code, WordPress
I only now stumbled on this question, so will copy-paste my response from the forum:

I am the developer of civicrm-region-lookup. I use it on CiviCRM 4.6, and it would require a small fix for CiviCRM 4.7, but it should work otherwise.

Although it is probably not the most user-friendly solution. It was meant to be flexible and vendor-neutral (i.e. not rely on specific paid sites for the data, make it possible to manage your own mappings).

For the US, you would need to import a list of postcodes via phpmyadmin, then configure the correct mapping in the admin settings (i.e. to set the field names so that the postcode field sets the city, but you might need an additional custom Javascript hook to set the state as well, which is not ideal, but not very complicated either).

There may be simpler solutions out there, especially for the US. If you decide to use this extension, I will be happy to provide some support on this forum if you run into issues.

Reference: https://forum.civicrm.org/index.php?topic=37098.0

See also the comments from petednz on the forum.