Lampcms is an full-featured Open Source Question and Answers web program with deep integration with Facebook, Twitter, Tumblr and Blogger and soon LinkedIn APIs.
Question2Answer (Q2A) is a popular open source Q&A solution, currently in use on 5,240 sites in 40 languages. It is built on the standard PHP/MySQL platform, and runs safe and fast.
3. Askbot
Askbot - the open source Q&A Forum
4. shapado
stackoverflow-like app written in ruby, mongomapper and mongodb
5. OSQA
OSQA is the free, open source Q&A system you've been waiting for. Your OSQA site is more than just an FAQ page, it is a full-featured Q&A community. Users earn points and badges for useful participation, and everyone in the community wins.