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.

Selectize.js

Selectize is a jQuery-based custom UI control. Useful for tagging, contact lists, country selectors, etc.

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.

Chained

Simple chained selects. You can choose from two different versions. Normal version uses uses classnames of select options to decide content of child select. It does not make any external AJAX(J) queries. Remote version makes external query and builds the child select from returned JSON response.

Selecter

A jQuery plugin for replacing default select elements. Part of the Formstone Library.

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 Selectric

Fast, simple and light jQuery plugin to customize HTML selects