WP Answers is a premium plugin that also includes its own responsive theme if required.
Unlike most of the other plugins, WP Answers doesn’t go for the Stackoverflow look and feel but provides a simpler but equally pleasing layout.
The plugin includes a reputation system, social media integration and built-in moderation. It also includes the ability to automatically populate with content from Yahoo Answers which is an interesting approach to encouraging those first tentative questions.
Currently there are two add-ons: PayPal which enables charging for posting a question and User Levels which is a simple permission system. The add-ons are only available with the developer licence.