We built the DO community site from scratch as there was nothing else out there that offered the tight integration we wanted between presenting content and offering a place to ask questions and discuss. Even though it didn't meet all of our requirements, we're big fans of Discourse. It's a great platform for hosting discussions.

If you're looking for something that follows the Q&A model, there are quite a few StackOverflow inspired sites out there:

http://meta.stackexchange.com/questions/2267/stack-exchange-...