DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
talkatv - talkatv is an open source comment service much like ...
Geist.chat - Add feature-rich responsive comment sections to your website.
Commentics - Commentics is an advanced PHP comment script with many features.
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