The following are the libraries I considered, and I find the first one, Ext JS, the most useful for developing a better User Interface.
As a client-side framework, Ext can run against any server platform that can process POST requests and return structured data. Common choices include PHP, Java, .NET and many more. There are server-side frameworks available for most popular platforms that will make your life much easier when programming under the AJAX application model that Ext uses. Ext has an open source LGPL license along with a per-developer, royalty-free commercial license.