Moot, which is really pretty and really fast (it’s also managing spam via Automattic’s Akismet) and is currently Free.
It was easy to start and very attractive. The company has a pretty extensive pedigree of founders to boot:
The company was founded by Portland-based serial entrepreneur Courtney Couch, along with jQuery Tools creator Tero Piirainen and Janne Lehtinen, both of Helsinki. The team had previously worked on popular web video player Flowplayer, which is what led them to the idea to create Moot in the first place.
The platform allows site owners to design their systems to function however they want.
Maybe you only need commenting, or maybe you want commenting and forums, or maybe you want some comments to appear on the forums. Maybe you want flat commenting. Or threaded. And maybe you want users to be able to comment using the same username and password they’re already using on your service today.
Just getting the basic forum up-and-running takes under a minute. Then it’s only a matter of copy-and-pasting some embed code.
Techcrunch review here
More into at moot.it