Right I would second Dojo, one thing to remember with Ext is they switched their licensing between versions, so if you are releasing a commercial product, you w

Right I would second Dojo, one thing to remember with Ext is they switched their licensing between versions, so if you are releasing a commercial product, you will have to make available your source code or pay for alternative licensing. It may not be an issue for your project, but there are a good deal of quality liberally licensed toolkits in this space such as Dojo, YUI and MooTools. For me personally I tend to use Dojo or a bolt together pattern of (Require, Backbone, jQuery, jQuery UI widgets both free and commercially sold ones).