Thredded is a Rails 4.2+ (yes! - it’s ready for Rails 5) forum/messageboard engine. Its goal is to be as simple, feature rich, and approachable as possible.
By “simple” we mean that it is a quick and easy way to extend an existing rails app with an forum engine instead of spinning up a whole other app like discourse - no docker needed, no extra heroku dynos, no knowledge of ember needed. It is easily theme'able, configurable, well tested, and has a long history with its maintainers (15 years and many rewrites/evolutions).