Question2Answer - A Q&A site helps an online community to share knowledge.
OSQA - OSQA is the free, open source Q&A system you've been waiting for.
Stack Overflow - Community-based Q&A part of the Stack Exchange platform.
The Answer Bank - The Answer Bank is the UK’s leading question-and-answer site.
AnswerHub - AnswerHub software helps developers capture, organize, and share their knowledge for greater productivity, innovation, and user satisfaction.
Niko - Niko is an employee experience, empowerment, and engagement solution.