I recently came across this plugin that really improves the usability of large select lists and multiple select form elements. The plugin is called Chosen and is available to use with JQuery, Prototype and Mootools.
Chosen offers enhanced features to select lists for searching through the options and supports multiple select lists and option grouping. Its most useful feature for me is for the multiple select lists, which is something I’ve always avoided using due to the fact that users rarely realise they can select multiple options using the control key.
Its a slick plugin that will be very useful for administration areas for example if items can be listed in multiple categories. When an option is selected its also removed from the list and offers change event triggers etc.
Download it for yourself: http://harvesthq.github.com/chosen/
Also very useful, now we are starting to use the Drupal CMS, is that Chosen is available as a Drupal 7 module. Download here: http://drupal.org/project/chosen
Below is an iframe showing a mini demo of the JQuery version.