Chosen is a javascript plug-in makes long, unwieldy select boxes much more user-friendly. It is currently available in both jQuery and Prototype flavors. Instead of forcing your users to scroll through a giant list of items, they can just start typing the name of the item they were looking for.

Add Chosen’s files to your app and then add the class chzn-select to your select box. Chosen automatically respects optgroups, selected state, the multiple attribute and browser tab order. You don’t need to do anything else except customize the style as you see fit.

Requirements: jQuery or Prototype Framework
Demo: http://harvesthq.github.com/chosen/
License: MIT License