Chosen

Chosen is a javsacript plug-in for Prototype (jQuery support coming soon) that makes long, unwieldy select boxes much more user-friendly.

Standard Select

Turns This

Into This

Turns This

Into This

Easier to Use

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.
Progressive Enhancement

Because chosen replaces normal html select fields, you don't need to do anything special to make it work for browsers without javascript.
Lightweight

Even un-minified, Chosen's JS, CSS and Images weigh in at under 50k.
Painless Setup

Add Chosen's files to your app and then add the class chzn-select to your select box. That's it, you're done!

Using Chosen is easy as can be.