DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Commento - A fast, bloat-free comments system to foster discussion on your website
IntenseDebate - IntenseDebate's comment system enhances and encourages conversation on your blog or website.
Vuukle - Vuukle is a multi-point user engagement platform.
XCommentPro - XCommentPro is a commenting and rating software.
Facebook Comments - Facebook comments plugin lets people comment on content on the site using their Facebook account.