DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Remarkbox - Remarkbox offers a free comment system to keep readers engaged with your content.
Commento - A fast, bloat-free comments system to foster discussion on your website
Commentbox - Commentbox is a jsf component that allows quick realization of a comment functionality
utterances - A lightweight comments widget built on GitHub issues.
Joint Comments - An embeddable comment widget for websites, blogs, ecommerce & static pages.