I found Google Wave to be somewhat impressive. It uses GWT.
There's a presentation about it's implementation using GWT here (I haven't watched it): http://code.google.com/events/io/2009/sessions/GoogleWavePow...
There's a presentation about it's implementation using GWT here (I haven't watched it): http://code.google.com/events/io/2009/sessions/GoogleWavePow...
The front end technology is definitely cool, but the UI sucks.