DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Question2Answer - A Q&A site helps an online community to share knowledge.
Answerbase - Add a Q&A system to your website in just minutes, with Answerbase's powerful question and answer software for online communities and customer support.
The Answer Bank - The Answer Bank is the UK’s leading question-and-answer site.
utterances - A lightweight comments widget built on GitHub issues.
Stack Overflow - Community-based Q&A part of the Stack Exchange platform.