These projects differ in their maturity, the extension of technologies they cover or the specialized server-side language they target. You will find below a list of some open source projects worth tracking down:

• AjaxAnywhere is designed to turn any set of existing JSP or JSF components into AJAX-aware components without complex JavaScript coding.
• Dojo is the Open Source JavaScript toolkit that helps you build serious applications in less time.
• Rialto (Rich Internet Application Toolkit) is AJAX-based cross browser JavaScript widgets library. It can be encapsulated in JSP, JSF, .Net or PHP graphic components.
• Sajax is an open source tool to make programming websites using the AJAX framework
• Ajax.NET is an implementation of the AJAX technology for the .NET environment.
• Rico provides a very simple interface for registering AJAX request handlers as well as HTML elements or JavaScript objects as AJAX response objects
• qooxdoo is an advanced open-source JavaScript based toolkit.
• CPAINT (Cross-Platform Asynchronous INterface Toolkit) is a multi-language toolkit that helps web developers design and implement AJAX web applications with ease and flexibility
• AjaxAC is an open-source framework written in PHP, used to develop AJAX applications.