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.
coComment - coComment is a service for managing, powering and researching conversations online.
IntenseDebate - IntenseDebate's comment system enhances and encourages conversation on your blog or website.
komentify - Enrich your website with comments in less than 30 seconds
RbComment - RbComment - Zend Framework 2 module for drop-in, self-hosted comments, with email notifications...