DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
JLex Comment - JLex Comment is a comment system for Joomla.
Commento - A fast, bloat-free comments system to foster discussion on your website
Vuukle - Vuukle is a multi-point user engagement platform.
HashOver - HashOver is a PHP comment system intended as a replacement for services like Disqus.
XCommentPro - XCommentPro is a commenting and rating software.