For a community forum, once upon a time, when PHP ruled the web universe, free & open-source forum software like phpBB and a premium software, vBulletin were the #1 choice. But things have changed quite recently in 2014 when one of the co-founders of StackOverflow, Jeff Atwood, launched his own version of a community forum software, Discourse, as open-source based on Ruby, Ember & PostgreSQL. Though I have no expertise in Ruby, Ember or Postgres, the installation ($150 one-time) & setup ($10 or more a month on DigitalOcean) can be arranged at a premium for clients wanting a serious online Q&A discussion platform.