DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
IntenseDebate - IntenseDebate's comment system enhances and encourages conversation on your blog or website.
coComment - coComment is a service for managing, powering and researching conversations online.
Commentbox - Commentbox is a jsf component that allows quick realization of a comment functionality
komentify - Enrich your website with comments in less than 30 seconds
Commento - A fast, bloat-free comments system to foster discussion on your website