Linb
- Provides an Object Oriented framework supporting AJAX and multilingual interface. Discussion group, download and documentation.
- http://code.google.com/p/linb/

Ext
- Provides an AJAX library and UI components.
- http://extjs.com/

Mootools
- Offers a lightweight web2.0 JavaScript framework.
- http://mootools.net/

Behaviour
- Offers a script allowing the use of css selectors applying the language behaviours.
- http://bennolan.com/behaviour/

MochiKit
- Provides a suite of libraries taken from Python. Documentation and download.
- http://www.mochikit.com/

Open Rico
- Offers visual effects, styling, drag and drop and ajax functions. Requires Prototype.
- http://www.openrico.org/

jQuery
- A fast, concise, library that simplifies how to traverse HTML documents, handle events, perform animations, and add AJAX.
- http://jquery.com/

Prototype
- Provides class-style Object Oriented and AJAX, freely distributable under the terms of an MIT-style license.
- http://www.prototypejs.org/

Script.aculo.us
- A collection of Web 2.0 style JavaScript libraries that help web developers to easily add visual and ajax effects to projects. Requires Prototype.
- http://script.aculo.us/