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 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.

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.

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.