Yahoo! Developer Network has a nice Valentine's present for all web developers – an open source BSD-style licensed UI library for creating fancy Web 2.0 applications – the

Yahoo UI Library

This is a set of utilities and controls, written in JavaScript, for building richly interactive web applications using techniques such as DOM scripting, HTML and AJAX.

So the Yahoo UI Library Utilities facilitates the implementation of rich client-side features by enhancing and normalizing the developer's interface to important elements of the browser infrastructure (such as events, in-page HTTP requests and the DOM).

The Yahoo UI Library Controls produce visual, interactive user interface elements on the page with just a few lines of code and an included CSS file.

Nice widgets they have are

Calendar

Slider

Tree View

Checkout those live samples… pretty nice…

The Yahoo design patterns library is a second goodie – for getting ome advice on implementation bascially… well, just patterns to reuse.

Yahoo's best visual and interaction designers have gotten together and created a set of patterns and guidelines for design in a Web 2.0 world. Here you'll find ideas on the best ways to implement web site elements, along with pointers to examples and the tools within the Yahoo! UI Library that will let you do it.