Legacy of Heroes is a Facebook app which incorporates an unusual cooperative combat system. While it is far from complete and a number of core concepts do not work as well in practice as on paper, the application has performed well as a test-bed for teaching myself about the Facebook API and for trying out certain design ideas without fear of commercial failure.
The pre-alpha application can be found at http://apps.facebook.com/legacy_of_heroes/ and should currently be playable. A changelog details the development of the concept, but it should be noted that this project is more of an experimental environment that a planned game and has no underlying design documentation.