DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Hyvor Talk - The Best Commenting Platform For Your Website
IntenseDebate - IntenseDebate's comment system enhances and encourages conversation on your blog or website.
Schnack - Schnack is an open source commenting system written in JavaScript.
Commentbox - Commentbox is a jsf component that allows quick realization of a comment functionality
Joint Comments - An embeddable comment widget for websites, blogs, ecommerce & static pages.