DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Stack Overflow - Community-based Q&A part of the Stack Exchange platform.
Commento - A fast, bloat-free comments system to foster discussion on your website
The Answer Bank - The Answer Bank is the UK’s leading question-and-answer site.
utterances - A lightweight comments widget built on GitHub issues.
Askalot - Askalot is a next generation CQA (Community Question and Answer) system.