This industrial strength JavaScript library was built by Yahoo! front end engineers and global contributors. Offering a scalable, fast and robust library with complete set of JavaScript, CSS utilities and controls, YUI is another free library developed under BSD license. YUI comes in handy to help developers build rich and interactive web apps using DOM scripting, DHTML and AJAX techniques.
Very nice AJAX based UI widgests are YUI - Yahoo user interface kit, very clean design, skinnable. Google has own, of course. Good web app framework (like TurboGears) simplifies AJAX/Jason interaction a lot.
One very interesting item that we discovered about the YUI Lightbox selection was that unlike the other javascript libraries, developers using the YUI library (for the most part) seem to be oblivious to the thought that other developers may have implemented lightbox functionality for the library already. This has led to an amazing lack of variety in the names of these different Lightbox Clones. I also believe that it's important to consider the very apparent differences in the composition of the community. As you noted, whereas YUI community members generally tend to be building JavaScript-heavy web applications, jQuery tends to attract a lot of designers. That difference tends to change both the essence of the questions as well as the number of questions and amount of involvement per user.
One thing that I have always loved about YUI is the very thorough documentation. Many times, I have dug through jQuery's plugin library just to find that a plugin site has been taken down... so what if I was already using that on a site? I'd be stuck or re-writing for another plugin. Not a situation you want to be in on a large site.
