This document discusses MVC Gems, including Inboxes and Polemic which provide private and group messaging and commenting abilities for Rails applications. It notes the requirements of Rails 3.1 or greater, Devise, and CanCan and describes Rails engines which contain their own models, controllers, views, routes, and other components to encapsulate functionality. Challenges with testing features interacting with Rails and controller flexibility from engines are addressed. Tools like Bundler, Travis CI, and documentation are suggested to support open source gems.