Chosen

Chosen is a JavaScript plugin that makes long, unwieldy select boxes much more user-friendly. It is currently available in both jQuery and Prototype flavors.

Select2

Select2 is a jQuery based replacement for select boxes. It supports searching, remote data sets, and infinite scrolling of results.

jQRangeSlider

A javascript slider selector that supports dates

Mobiscroll Select Scroller

Renders a scrollable list from HTML selects for easy data picking on touch devices. It comes with theming capability, optional jQuery Mobile integration and multiple display modes: Inline, Modal, Bubble, Docked Top & Bottom. It works with optgroups as well.

ImgAreaSelect – image selection/cropping plugin

ImgAreaSelect is a plugin for selecting a rectangular area of an image. It allows web developers to easily implement image cropping functionality, as well as other user interface features (such as photo notes).

jQuery multiselect

This is a tiny jQuery plugin which aims to be a drop-in replacement for the standard <select> element with multiple attribute activated.

Selecter

A jQuery plugin for replacing default select elements. Part of the formstone library.

jQuery Flexselect

jQuery plugin that turns select boxes into flex-matching incremental-finding controls.

Marco Polo

A jQuery autocomplete plugin for the discerning developer.