There are so many web frameworks out there that one doesn't even know which one is worth exploring. I've recently seen an amazing demo app created with Echo2. Check it out here. There is also demo webmail, chat and other demos worth clicking on.
One thing that I found really interesting about this framework is that the whole gui is composed from java components (like Swing) that are rendered as xhtml/css/js, and all the communication between client and server is done via AJAX. This makes the app responsiveness heavily dependent on the network latency but that it is the only price you pay for using this framework.