Question2Answer is a free and open source platform for Q&A sites.
A Q&A site helps your online community to share knowledge. People with questions get the answers they need. The community is enriched by commenting, voting, notifications, points and rankings.
Your members will enjoy the interaction that Q&A enables, and will visit your site more regularly. In addition, many web searches are questions, so Q&A content will attract search engine traffic.
Question2Answer (Q2A) is a popular open source Q&A platform for PHP/MySQL, currently running on 22,709 sites in 40 languages.