JLex Comment - JLex Comment is a comment system for Joomla.
DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Vuukle - Vuukle is a multi-point user engagement platform.
IntenseDebate - IntenseDebate's comment system enhances and encourages conversation on your blog or website.
XCommentPro - XCommentPro is a commenting and rating software.
Commentbox - Commentbox is a jsf component that allows quick realization of a comment functionality