DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
LiveFyre - livefyre-subscriptions - A isomorphic JavaScript library for managing Subscriptions in Livefyre's Personalized News Stream service.
Commento - A fast, bloat-free comments system to foster discussion on your website
IntenseDebate - IntenseDebate's comment system enhances and encourages conversation on your blog or website.
utterances - A lightweight comments widget built on GitHub issues.
RbComment - RbComment - Zend Framework 2 module for drop-in, self-hosted comments, with email notifications...