Question2Answer - A Q&A site helps an online community to share knowledge.
Vanilla Forums - Build an engaging community forum using Vanilla's modern cloud forum software.
Stack Overflow - Community-based Q&A part of the Stack Exchange platform.
Discourse - Discourse is an open source discussion platform built for the next decade of the Internet.
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.
XenForo - Intuitive. Social. Engaging. Fast. XenForo brings a fresh outlook to forum software.