I checked this out when researching a few frameworks for a new application. Even though it was an ASP.Net project we decided to go with Dojo http://dojotoolkit.

I checked this out when researching a few frameworks for a new application. Even though it was an ASP.Net project we decided to go with Dojo http://dojotoolkit.org http://dojotoolkit.org which I feel has a better product and model. It's free but if you need professional support you can get it through Sitepen.com. I've been impressed with it so far.