iCarousel is an open source JavaScript tool for creating carousel like widgets.  Based on the MooTools JS library, iCarousel is a nice widget with lots of possibility for customization.

Prototype 1.5: The Complete Documentation has just been released. This 166 page pdf e-book was written by Sam Stephenson and the Prototype team so should be an authoritative source of docs.

Mapstraction is a javascript library that hides differences between mapping APIs. It provides a common set of JavaScipt functions that lets you switch seamlessly between Google, Yahoo! and Microsoft’s javascript mapping APIs.

Jack Slocum has created an excellent reusable component for resizing elements. The component uses his custom version of the YUI JavaScript library and is very easy to include an use in your scripts. Be sure to scroll down his page to check out some very cool examples.

Mootools is a small object-oriented JavaScript library that is crossbrowser compatible and easy to use. Some features include DOM effects, Ajax, DOM Navigator, drag and drop, sortable lists, cookies management and much more. The framework also makes it easy to extend HTML elements with your own methods, to help make your coding style way cooler.

One of the nicer features is the way you download mootools. You choose the features you want to use, and mootools packages a custom JS file that includes all dependencies. This makes mootools very small and efficient!

Mike Arace shows us how to create a CSS Crossfader Demo using CSS, the Scriptaculous JavaScript effects library and a bit of semantic markup!

Low Pro is an unobtrusive scripting library build ton top of Prototype. It provides a bunch of neat methods that ease DOM manipulation and methods for easing the creation of nodes in the DOM.

Here is a nice two part interview with Alex Russell who is the creator of the Dojo AJAX toolkit. In Part I, Alex discusses the history and strengths and weaknesses of Dojo. Part II looks at AJAX challenges as well as competition to the Dojo toolkit.

The Ajaxian blog has a great online tutorial on using the Dojo Toolkit. In the lesson you will learn all about that Dojo thing, setting up Dojo, DOM and HTML effects, AJAX remote calls via dojo.io.bind(), and drag and drop operations.

Here is great guide for using prototype.js v1.4.0. This site is full of great help and is probably the best online documentation for the Prototype JavaScript library.