Javascript libraries

Currently, several comprehensive Javascript libraries are receiving attention from web developers for Web 2.0 "richly interactive features" and AJAX implementation. Also, they insulate the web developer from the erratic and "non-standard" behaviour of the popular browsers. Below is a typical example of tabs providing access to different content. The content associated with each tab fades smoothly in and out as tabs are clicked. These were created using the "Tabs" plugin for jQuery from Klaus Hartl These tabs record usage in browser history so the "back button" behaves as a user would expect (the jQuery plugin for this is "history_remote").

Jquery from John Resig and the jquery team. I use jQuery because it is packaged with the CMS I use (Drupal). I wrote a module to only include the plugin and associated css on specified pages.
MochiKit, a lightweight Javascript library. The slogan on their website is "MochiKit makes Javascript suck less", an interesting view, faint praise . . . !
Dojo is an Open Source DHTML toolkit written in JavaScript. It builds on several contributed code bases (nWidgets, Burstlib, f(m)), which is why we refer to it sometimes as a "unified" toolkit. Dojo aims to solve some long-standing historical problems with DHTML which prevented mass adoption of dynamic web application development.

Their website is Drupal too, jquery and dojo playing together!
Prototype is a JavaScript Framework that aims to ease development of dynamic web applications. Featuring a unique, easy-to-use toolkit for class-driven development and the nicest Ajax library around, Prototype is quickly becoming the codebase of choice for web application developers everywhere.

Its development is driven heavily by the Ruby on Rails framework, but it can be used in any environment.
The Yahoo! User Interface (YUI) Library is a set of utilities and controls, written in JavaScript, for building richly interactive web applications using techniques such as DOM scripting, DHTML and AJAX. The YUI Library also includes several core CSS resources. All components in the YUI Library have been released as open source under a BSD license and are free for all uses.

Their website has fantastic video presentations from Douglas Crockford and others on Javascript, the DOM and browsers in general, I recommend them.
I guess all Javascript programmers are interested to know what libraries are being used. If you use one of these libraries, please vote for one listed below. If I have omitted a popular library, please drop me an email and I will add it.

You can see the results of this poll here.