A social Q&A site encourages your online community to share knowledge and helps create content that is very search engine (SEO) friendly. Question2Answer is a free PHP/MySQL solution for social Q&A sites (like Stack Overflow or Stack Exchange) with the following features:
• Five minutes (or less!) to install.
• Easy styling with CSS themes.
• Fast integrated search engine.
• Points-based reputation management.
• Categories and/or tags (with auto complete).
• Email notifications and RSS support.
• Voting, comments and follow-on questions.
• Simple and secure admin interface.
• PHP/MySQL scalable to 1,000,000+ posts.
• Built-in user account management, or...
• Single sign-on for existing sites.