DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Commento - A fast, bloat-free comments system to foster discussion on your website
utterances - A lightweight comments widget built on GitHub issues.
UserVoice - UserVoice integrates easy-to-use feedback, helpdesk, and knowledge base management tools in one platform that empowers users to speak and companies to understand.
JustComments - Easy to set up, ad-free and reliable comment system
Cusdis - Open-source, lightweight comment system for your website