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.
Commento - A fast, bloat-free comments system to foster discussion on your website
JLex Comment - JLex Comment is a comment system for Joomla.
Commentbox - Commentbox is a jsf component that allows quick realization of a comment functionality
Facebook Comments - Facebook comments plugin lets people comment on content on the site using their Facebook account.