Last week a stable release of SerfJ (http://serfj.sourceforge.net) has been launched. SerfJ is an acronym of Simple Ever REST Framework for Java. Its main purpose is to provide a lightweight MVC arquitechture based on REST-like URLs.
It has a minimal configuration, you can make a J2EE application with only one line within its configuration file, along with some lines for mapping URLs within web.xml.
Apart from serving .JSP or .HTML pages, controller's methods are able to return serialized objects as JSON or XML, although developers are free to develop their own serializers (XML, JSON, CSV, or whatever they need).
This project is opensource under Apache 2.0 license, hosted at Sourceforge. Also its artifacts can be found in Maven Central repository.
You can find more information about how to use it at http://serfj.sourceforge.net, the development team is looking forward to getting your feedback.