Cinammon Boards is a WordPress forum plugin with a light footprint, responsive structure and user accessibility. The forum uses custom post types and all information is structured as a forum (categories, forums and topics). It can easily be integrated into any theme without losing any of its functionality.
Cinnamon Boards acts like a real forum, with categories, forums, topics, threads and messages, but it is heavily integrated into WordPress, taking advantage of the user/author system, Akismet and theme blending.
Cinnamon Boards works out of the box, with no options to set, except for the creation of categories and forums.
Demo #1 http://cghubs.com/forums/
Demo #2 http://getbutterfly.com/forums/