Vinish Garg
Vinish Garg reviewedDiscourseA platform for community discussion. Free, open and simple.
 RecommendedPros: 

Highly customizable and modular in what exactly we need.

Cons: 

Bit of a learning curve while setting up first time. :)

I have used it extensively in Andrew Chen's talk platform: http://talk.andrewchen.co/ and also for Write The Docs forum at http://forum.writethedocs.org/. I think it meets most of the goals of a well structured, well monitored, and custom discussion forum. I like it.