DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
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.
Commento - A fast, bloat-free comments system to foster discussion on your website
utterances - A lightweight comments widget built on GitHub issues.
JustComments - Easy to set up, ad-free and reliable comment system
Question2Answer - A Q&A site helps an online community to share knowledge.