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.
IntenseDebate - IntenseDebate's comment system enhances and encourages conversation on your blog or website.
Cusdis - Open-source, lightweight comment system for your website
coComment - coComment is a service for managing, powering and researching conversations online.