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
IntenseDebate - IntenseDebate's comment system enhances and encourages conversation on your blog or website.
RbComment - RbComment - Zend Framework 2 module for drop-in, self-hosted comments, with email notifications...
Commentbox - Commentbox is a jsf component that allows quick realization of a comment functionality
coComment - coComment is a service for managing, powering and researching conversations online.