SabaiDiscuss is a premium questions and answers plugin for WordPress. The plugin features the ability for users to ask and answer questions similar to Stack Overflow or Yahoo Answers. SabaiDiscuss is an ideal tool not only for building a community driven question-and-answer website but also for building a discussion forum, a knowledge base, or even a helpdesk portal for WordPress.

Demo: http://codecanyon.net/item/sabai-discuss-for-wordpress/3455723