The Tandem Select jQuery Plugin implements tandem, or two stage, select menus in web forms using jQuery and JavaScript.
The code can be downloaded from GitHub: https://github.com/craic/tandem_select_jquery_plugin
More information and demos can be found on the project page: http://craic.github.com/tandem_select_jquery_plugin/
The plugin in was written by Rob Jones of Craic Computing in Seattle.