I came across this article on top ten obscure but useful Java Open Source projects. Along with the project listing, they article also describes why it is useful. Read the article here.
It is good to find Queplix there. It is open source, customer care application, which leverages GWT for UI. They did a fantastic job although I feel that the UI transition is not really smooth.Check their demo here. The user name and password for the demo is ‘demo’.