Discourse

The open-source forum engine behind thousands of dev, gaming, and support communities.

Best for: Teams that want a dedicated, deeply-featured public forum or support community and are fine owning the Rails/Postgres/Redis stack behind it.
Skip if: You just need a lightweight embeddable comments widget, or you don't want to operate Ruby/Rails infrastructure.

Discourse is a self-hostable community platform built on Rails and Ember, combining classic forum threads with modern touches like real-time updates, trust-level based moderation, and a large plugin/theme ecosystem. It's free and fully open source (GPLv2) to run yourself, or available as a managed hosted product with paid tiers for larger communities.

Pros

Mature, battle-tested forum engine running thousands of large communities for over a decade
Fully open source under GPLv2 - self-host for free with no feature gate
Deep plugin/theme ecosystem plus real-time updates, trust levels, and moderation tooling out of the box

Cons

Rails + Redis + Postgres stack adds real operational overhead to self-host well
It's a full standalone site, not a drop-in embed for an existing app
Hosted plans jump from free to $100/mo (Pro) to $500/mo (Business) fast as a community grows