Google's Ajax Libraries API

I don't use a lot of JavaScript, but as you probably know, I'm a fan of offloading things to other people when it can save me the hassle of doing it myself.
A CDN — short for Content Delivery Network — distributes your static content across servers in various, diverse physical locations. When a user’s browser resolves the URL for these files, their download will automatically target the closest available server in the network.

Dropdown menus and menu bars have been heavily used since the early days of graphical user interfaces. Their use has become ubiquitous, and even expected, in desktop applications, and the web has quickly followed suit. There are lots of dropdown menus already out there. Here you’ll find 25 sexy jQuery and CSS based drop-down or just multi level menu tutorials with down loadable files and explanations as well.

jQuery is a fast and concise JavaScript Library that simplifies HTML document traversing, event handling, animating, and Ajax interactions for rapid web development. jQuery is designed to change the way that you write JavaScript.
jQuery is a lightweight JavaScript library that emphasizes interaction between JavaScript and HTML. It was released in January 2006 at BarCamp NYC by John Resig.
jQuery is free, open source software Dual-licensed under the MIT License and the GNU General Public License.
jQury is available in two formats:

• Compressed (which allows you to have a significantly smaller file size) a.k.a Minified version.
• Uncompressed (good for debugging and to understand what is behind the magic).

The minified versions, while having a larger file size than the packed versions (note: packed version is not available in current release), are generally the best versions to use on production deployments. The packed versions require non-trivial client-side processing time to uncompress (unpack) the code whereas the minified versions do not. The packed versions of jQuery will take less time to download than the minified or uncompressed versions; however, each time the library is loaded (initially or from the browser cache) it will need to be uncompressed which will cause a non-trivial delay in the execution of any jQuery code each time it is loaded.

YUI Library

The YUI Library is a set of utilities and controls, written with JavaScript and CSS, for building richly interactive web applications using techniques such as DOM scripting, DHTML and AJAX. YUI is available under a BSD license and is free for all uses.
Development on YUI began in 2005 and Yahoo! properties such as My Yahoo! and the Yahoo! front page began using YUI in the summer of that year. In February 2006 YUI was released for public use under BSD. It is actively developed by a core team of Yahoo! engineers.
YUI is proven, scalable, fast, and robust. Built by frontend engineers at Yahoo! and contributors from around the world, it's an industrial-strength JavaScript library for professionals who love JavaScript.

There are currently two versions of YUI: YUI2 and YUI3. Both are proven, scalable, fast, and robust. The version of YUI that is best for you depends entirely on your needs and project requirements. The following provides some guidance on choosing between the versions of YUI.