ExtJs (extjs.com) is a excellent javascript library for building rich UI interactive web applications using Ajax technology. The following C# ASP .net wrappers are availalble for the ExtJS library.

i. Coolite (coolite.com)

ii. Ext Extender toolkit (www.codeplex.com/ExtJsExtenderControl)

iii. Ext Sharp (code.google.com/p/extsharp/)