CM Answers (answers.cminds.com) is a great WordPress plugin for anyone who wishes to encourage user interaction on their WordPress site. It allows users to post questions, answers, and comments in a Q&A discussion forum in the style of StackOverflow. Our Question and Answer discussion forum plugin also includes social media registration using Facebook, LinkedIn or Google+ login. The CM Answers Q&A plugin has many other unique features, which allow administrators to customize the display of visitors' questions and rank comments and answers according to popularity or number of votes. Available for free download!
More info and free download on answers.cminds.com