Yahoo! UI Library (YUI) is an open-source JavaScript library, for building richly interactive web applications using techniques such as Ajax, DHTML and DOM scripting. It also includes many CSS resources. It is available under BSD License.

Features

YUI Library comes with complete documentation. It has two types of components: Utilities and Controls, and some CSS resources.

Utilities

AnimationHelps create "effects" by animating the position, size, opacity or other characteristics of page elements.
Browser History ManagerHelps web applications use the browser's back button and bookmarking functionality.
Connection ManagerHelps manage XMLHttpRequest transactions in a cross-browser fashion. It has integrated support for form posts, error handling, callbacks and file uploading.
DataSourceProvides a common configurable interface for other components to interact with different types of data, from simple JavaScript arrays to online servers over XHR.
ElementProvides a wrapper for HTMLElements in the DOM and makes simpler common tasks such as adding listeners, manipulating the DOM, and setting and getting attributes.
DOMHelps with common DOM scripting tasks, including element positioning and CSS style management.
Drag and DropHelps with Drag-and-drop development (creation and management of draggable objects that can be picked up and dropped elsewhere on a web page)
EventProvides developers with easy and safe access to browser events (such as mouse clicks and key presses). It also provides the Custom Event object for publishing and subscribing to custom events.

Controls

AutoCompleteProvides autocomplete feature (suggestion lists and type-ahead functionality) for user interactions involving text-entry. It supports a variety of data-source formats. It also supports server-side data-sources via XMLHttpRequest.
ButtonEnables the creation of rich, graphical buttons that function like traditional HTML form buttons.
CalendarA graphical, dynamic control used for date selection.
ContainerSupports a variety of DHTML windowing patterns including Tooltip, Panel, Dialog, SimpleDialog, Module and Overlay.
DataTableSimple yet powerful API to display screen-reader accessible tabular data on a web page. Notable features include sortable columns, pagination, scrolling, row selection, resizeable columns, and inline editing.
LoggerProvides a quick and easy way to write log messages to an on-screen console, the FireBug extension for Mozilla Firefox, or the Safari JavaScript console.
MenuProvides easy way to create fly-out menus.
SliderProvides a generic slider element that enables the user to choose within a finite range of values on one or two axes.
TabViewProvides navigable tabbed views of content.
TreeViewProduces a content tree whose nodes can be expanded and contracted.

CSS resources

CSS Page Grids: Seven basic page wireframes with subsection components to support over 1000 different page layouts.

Standard CSS Fonts: Standardized cross-browser font families and size rendering.

Standard CSS Reset: CSS declarations remove margins and standardize cross-browser rendering on common elements.

In August 2007, Yahoo released YUI Compressor 1.0, a JavaScript compressor.

The YUI Theater helps provides worldwide access to several talks by notable speakers on JavaScript and web development.