AJAX .NET Professional (http://www.ajaxpro.info/)
Magic AJAX.NET (http://www.magicajax.net/)
Microsoft ASP.NET AJAX Extensions (http://ajax.asp.net)
Microsoft AJAX Toolkit (http://ajax.asp.net/ajaxtoolkit)
Prototype (http://prototype.conio.net/)
Scriptaculous (http://script.aculo.us/)