DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
SolidOpinion - SolidOpinion is a fun, new and dynamic commenting platform.
Joint Comments - An embeddable comment widget for websites, blogs, ecommerce & static pages.
Commentbox - Commentbox is a jsf component that allows quick realization of a comment functionality
Hyvor - Hyvor is a privacy-focused and fully-featured commenting system created for almost all kinds of web applications and blogs.
IntenseDebate - IntenseDebate's comment system enhances and encourages conversation on your blog or website.