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.

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.

jQuery UI Autocomplete

Lists suggested words as the user is typing.

jQuery UI Datepicker

Displays a calendar from an input or inline for selecting dates.

jQuery UI Slider

Displays a flexible slider with ranges and accessibility via keyboard.

jQuery UI Button

Enhances a form with themeable buttons.

jQuery UI Spinner

Displays buttons to easily input numbers via the keyboard or mouse.

Fine Uploader

Multiple file upload library with support for all major browsers. Many features such as drag & drop, auto & manual retry, file chunking, auto-resume, and more! Does NOT use Flash or Java. Highly configurable. A no-dependency version is ALSO available in the downloads section.

mailcheck

The Javascript library and jQuery plugin that suggests a right domain when your users misspell it in an email address.

Selectize.js

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