DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Remarkbox - Remarkbox offers a free comment system to keep readers engaged with your content.
IntenseDebate - IntenseDebate's comment system enhances and encourages conversation on your blog or website.
Commentbox - Commentbox is a jsf component that allows quick realization of a comment functionality
Commento - A fast, bloat-free comments system to foster discussion on your website
Vuukle - Vuukle is a multi-point user engagement platform.