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.
Master Comments System - Master Comments System is a very compact comments system with basic options:
IntenseDebate - IntenseDebate's comment system enhances and encourages conversation on your blog or website.
utterances - A lightweight comments widget built on GitHub issues.
Commento - A fast, bloat-free comments system to foster discussion on your website