There are many Frameworks, which do you prefer?
1. script.aculo.us (http://script.aculo.us/)
Note: script.aculo.us is not a Framework by its self but it is an addon for Prototype.
2. Prototype (http://www.prototypejs.org/)
3. Moo Tools (http://www.mootools.net/)
4. jQuery (http://jquery.com/)
5. MochiKit ()
6. Rialto ()
7. Dojo Toolkit (http://dojotoolkit.org)
8. Spry Framework (http://labs.adobe.com/technologies/spry/)
9. ASP.NET Ajax Framework (http://asp.net/ajax/)
“ASP.NET AJAX is a free framework for quickly creating efficient and interactive Web applications that work across all popular browsers”.
10. Cean Ajax Framework (http://sourceforge.net/projects/clean-ajax/)
“Easy to use AJAX framework that provides message queue, XSLT, XPath, encryption (SHA1, MD5), web service access (SOAP, XMLRPC), JSON-RPC, cross browser AJAX, AJAX history and cache control”.