Reference field option limit

The Reference field option limit module allows reference fields of several types to have their widgets' available options limited by the values of other fields in the current entity.

A simple set-up of this allows you to have two fields in your entity form that work in tandem, with the first field refining the available options in the second field via AJAX. For example, you could select first a country, which causes the city field to refresh and show only the cities in the currently selected country.