Collected by my colleague Joe Stagner and Tim Heuer.
Microsoft Ajax - http://ajax.asp.net
Java BluePrints - https://blueprints.dev.java.net/
Protptype - http://www.prototypejs.org/
Scriptaculus - http://script.aculo.us/
Laszlo - http://www.openlaszlo.org/
GWT - http://code.google.com/webtoolkit/
AJAX.net - http://www.ajaxpro.info/
Yahoo - http://developer.yahoo.com/yui/
Active Widgets - http://www.activewidgets.com/
Tibco General Interface - http://developer.tibco.com/
Visual Web GUI - http://www.visualwebgui.com/
Dojo - http://dojotoolkit.org/
Backbase - http://www.backbase.com/
SmartClient - http://www.smartclient.com/
Nitobi - http://www.nitobi.com/
JackBe - http://www.jackbe.com
Zimbra - http://www.zimbra.com/
Rico - http://www.openrico.org/
Spry - http://labs.adobe.com/technologies/spry/
Aflax - http://www.aflax.org/
DWR - http://getahead.org/dwr
Moo.fx - http://moofx.mad4milk.net/
Mochikit - http://www.mochikit.com/
Xajax - http://www.xajaxproject.org/
Nexaweb - http://www.nexaweb.com
Behaviour - http://www.bennolan.com/behaviour/
AJForm - http://projects.aphexcreations.net/ajform/
Qooxdoo - http://qooxdoo.org/
Bindows - http://www.bindows.net/
Tibet - http://www.technicalpursuit.com/ajax_indepth.htm
Crossvision - http://www.softwareag.com/Corporate/products/cv/appldes/default.asp
Echo2 - http://www.nextapp.com/platform/echo2/echo/
Ajax Tags - http://ajaxtags.sourceforge.net/ (JSP based)
WebWork - http://www.opensymphony.com/webwork/
Tacos - http://tacos.sourceforge.net/ (for Tapestry)
Sajax - http://www.modernmethod.com/sajax/
AjaxAC - http://ajax.zervaas.com.au/
JSPan - http://sourceforge.net/projects/jpspan
HTML_AJAX - http://pear.php.net/package/HTML_AJAX/ (PHP)
Aspects - http://ajaxaspects.blogspot.com/
WebORB - http://www.themidnightcoders.com/weborb/
TurboGears - http://www.turbogears.org/
MagicAjax - http://www.magicajax.net/