Discourse is the world-leading online web-based forum. It's a superb, extremely mature-and-yet-cutting edge platform. It also happens to be Free and Open Source Software, which is why we, at the OER Foundation, use it.
Like all good software, it's undergoing continuous improvement by its developer community who release fairly frequent - perhaps every couple weeks - updates. Luckily, keeping your Discourse forum up-to-date isn't particularly onerous.
At the OERu we have two separate instances of market category leading Discourse Forum: one for OER partner and contributor collaborators and the other for learners. These days, online forums are seen as a bit old-school: fuddy-duddy. From my point of view, however, Discourse is "Forum-NG" (a Next Generation forum).