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.

jQuery UI Selectmenu

Duplicates and extends the functionality of a native HTML select element, allowing it to be customizable in behavior and appearance far beyond the limitations of a native select.

Selectize.js

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

MagicSuggest

MagicSuggest is a multiple selection auto suggest combo box for Bootstrap 3.

multiselect

This is a user-friendlier drop-in replacement for the standard with multiple attribute activated.

jQRangeSlider

A javascript slider selector that supports dates

Bootstrap Tags Input

jQuery plugin providing a Twitter Bootstrap user interface for managing tags.

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.