NodeBB

Github: https://github.com/NodeBB/NodeBB
HomePage: https://nodebb.org/
DemoSite：https://try.nodebb.org/

Discourse

Github：https://github.com/discourse/discourse
HomePage：https://www.discourse.org/
DemoSite：https://try.discourse.org/

Flarum

Github：https://github.com/flarum/flarum
HomePage：https://flarum.org/
DemoSite：https://discuss.flarum.org/

Vanilla

Github：https://github.com/vanilla/vanilla
HomePage：https://vanillaforums.com/en/software/
DemoSite：https://open.vanillaforums.com/