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 Menu

Creates nestable menus.

MagicSuggest

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

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.

Select or Die

Yet another jQuery plugin to style/replace select elements.

jQuery Selectric

Fast, simple and light jQuery plugin to customize HTML selects

jQuery Superfish Menu Plugin

Superfish is a jQuery plugin that adds usability enhancements to multi-level drop-down menus. Fully supports touch devices and keyboard interaction.

jQuery Flexselect

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

HeapBox

Customizable replacement for html select (not only) with events, callbacks, themes and much more.