GWT Portlets is a free open source web framework for building GWT applications. It defines a very simple & productive programming model to build good looking, modular GWT applications.
The programming model is somewhat similar to writing JSR168 portlets for a portal server (Liferay, JBoss Portal etc.). The "portal" is your application built using the GWT Portlets framework as a library. Application functionality is developed as loosely coupled Portlets each with an optional server side DataProvider.