We talked about a global architecture few posts ago. In this architecture, we pointed out GWT, OSGI, REST, PureMVC, DB4O as basis elements.
I've just read this morning the emergence of a new open source project : miniG, a GMail-like email application.
What is interesting in this project is the technical choices which has been made, ie GWT, OSGI and REST.
I enjoin you to take a glance to this project.