DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
IntenseDebate - IntenseDebate's comment system enhances and encourages conversation on your blog or website.
Commento - A fast, bloat-free comments system to foster discussion on your website
Thrive Comments - Thrive Comments is a comment system for Wordpress.
utterances - A lightweight comments widget built on GitHub issues.
GraphComment - Add modern discussion and social interactions to your website