Discourse

Discourse is a discussion platform or a modern take on the web forum. It has a very modular system built on top of rails and contains great examples of how to build an interactive frontend in ember within a rails application. It has an admin interface, signup/sign-in with Oauth for Google, Facebook, Twitter, Yahoo, and Github. It has extensive i18n, real time notifications, a plugin ecosystem, is SEO optimized, and is designed for tablet and mobile devices.