Activeselect

An activeselect element is the same as a regular select element, except that when the user selects a new option (or set of options), one or more different select elements (the target elements) get their lists updated.This is done using AJAX, and it is designed to degrade gracefully if the... Platforms: Windows, Mac, *nix, PHP, BSD Solaris

This is an add-on for the addresses_cck module. It provides a dropdown select field for the country and uses the addresses data to find the relevant state/provinces for the selected country. Also comes with the activeselect module for Drupal 6. Platforms: PHP