Didn't see this one mentioned, Vaadin (previously known is IT Mill Toolkit), but maybe that's because it's not technically exactly GWT; as their FAQ says:
How does this differ from GWT?
GWT applications run in the browser,
while Vaadin applications run on the
server. We do actually use GWT as a
"rendering engine" on the browser
side, so you can combine Vaadin and
GWT.
