Best server-side framework for heavy AJAX Java application

Echo2 / Echo3 by Nextapp (www.nextapp.com) is totally awesome.

Advantages over GWT:

1) It is not limited to a sub-set of java like GWT2) It is easier (in my estimation) to learn3) Has extremely robust design studio for almost drag and drop designing.
4) It is very fast, and works very well on all platforms browsers5) You can write your application using either java script or java6) It has great and straight forward methods for handling events and actions.

Personally I think that for any web-application in which you are trying to integrate java and speedy delivery I wouldn't hesitate to pick Echo3 or Echo2.

If you're starting from scratch. I'd have to say Google Web Toolkit. I have to say it is incredibly powerful. You get keep using most of your Java tools. Plus, you don't have to duplicate code that exists on both the server and the client, it just gets compiled differently for each area.

I'd consider REST-style frameworks as well as the other recommendations here- Restlet or Jersey may be good choices for the backend, while you use something like JQuery or GWT on the front end. Both frameworks can easily produce JSON, and the REST style provides a nice clean line of demarcation between your client application and your server source; I find that JSF can make that demarcation pretty muddy.