Yahoo! User Interface Library:

The Yahoo! User Interface Library 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. The UI Library Utilities facilitate 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. All the components in the Yahoo! User Interface Library have been released as open source under a BSD license and are free for all uses.

Yahoo! Design Pattern Library:

Welcome to the Yahoo! Design Pattern Library. We are very happy to be sharing our library with the design and development community. This is our first drop of what we hope to be a monthly release cycle for the publication of patterns. In many cases we have bundled the patterns with pointers to related code from the Yahoo! User Interface Code Library. We hope this is a useful resource and look forward to your feedback.

[Via Simon Willison]