Yep, discourse.org. It's a little too heavy weight for my uses and I think they really need to get the client and server requirements down a bit so that it can take over where PHPBB was once used.

See http://discuss.emberjs.com/ and http://bbs.boingboing.net/ for real world implementations. It's very, very cool to see forums being dragged out of the 1990s.

Jeff left StackExchange, he's just working on his own now.