Top 5 open source Q&A systems
http://www.mkyong.com/featured/top-5-open-source-qa-systems/


List of the open source Q&A implementation or stackoverflow-like website, with “Active” development and promising features and community.

P.S This list is not in any particular order.
1. OSQA
osqa

Website : http://www.osqa.net
Demo : http://meta.osqa.net
Language : Python, Django
Database : MySQL, PostgreSQL or Microsoft SQL Server
License : GNU General Public Licence (GPL)
Source code : http://www.osqa.net/download/
Created by : Folks at DZone

2. Shapado
shapado

Website : http://shapado.com
Demo : http://shapado.com
Language : Ruby , MongoMapper
Database : MongoDB (Document Based DB engine)
License : GNU Affero General Public License (AGPL)
Source code : http://gitorious.org/shapado
Created by : David A. Cuadrado

3. Askbot
askbot

Website : http://askbot.org
Demo : http://askbot.org
Language : Python, Django
Database : MySQL, PostgreSQL
License : GPL3
Source code : https://github.com/ASKBOT/askbot-devel/
Created by : Mike Chen and Sailing Cai

4. Question2Answer
question2answer

Website : http://www.question2answer.org
Demo : http://www.question2answer.org/qa/
Language : PHP
Database : MySQL
License : GNU General Public Licence (GPL)
Source code : http://www.question2answer.org/install.php
Created by : Gideon Greenspan

5. LampCMS
lampcms

Website : http://www.lampcms.com
Demo : http://support.lampcms.com
Language : PHP
Database : MongoDB
License : GNU LESSER GENERAL PUBLIC LICENSE (LGPL) version 3
Source code : https://github.com/snytkine/LampCMS
Created by : Dmitri Snytkine (Twitter)