Question2Answer - Stackoverflow styled, PHP base Q&A system

A Q&A site helps your online community to share knowledge. No matter what your area of interest, people with questions can get super answers fast. Refer to the demo page http://www.question2answer.org/qa/. It is open source but with some restrictions. Check out its license page

Its feature include

• Five minutes (or less!) to install
• Easy styling with CSS themes.
• Fast integrated search engine.
• Points-based reputation management.
• Question tagging (with auto completion).
• Comments on questions and answers.
• Simple and secure admin interface.
• PHP/MySQL scalable to 1,000,000+ users.
• Built-in user account management, or...
• Single sign-on for existing sites.