Stack Overflow - Community-based Q&A part of the Stack Exchange platform.
Question2Answer - A Q&A site helps an online community to share knowledge.
The Answer Bank - The Answer Bank is the UK’s leading question-and-answer site.
AnswersLOG - AnswersLog is a question-answer platform that provides unique features to help you to give answers to your questions in better ways and make your content more popular.
AskBot - Askbot enables users to create Q&A forums.
Niko - Niko is an employee experience, empowerment, and engagement solution.