Discourse is a forum/Q&A hybrid.
Discourse markets itself as a concoction of the good collaborative environment of old-school forums, and the interactivity of modern websites. It's also open-source, you can run a Discourse forum of your own.
Discourse also offers hosting for a very hefty price (~$100 per month). As such, it's very possible that anyone who's using their hosting for their forum could chicken out at any minute, meaning their forum is incredibly volatile.
- Imgur Community Forums