Dependent Dropdown is a jQuery plugin that enables to add and control multi level dependent dropdown lists. It works both with normal select options and select with optgroups as well.
Chained is simple jQuery plugin for chained selects. There are two different versions.
A simple and lighweight jQuery plugin for cascading dropdowns.
The plugin allows you to implement a cascading dropdown using multiple select boxes in which one select box depends on the input from one or more separate select boxes. Once one or all (depending on the configuration) required select boxes contain a value, it will trigger an event that will fire off the dependant select box so it will fetch its own list.