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
JustComments - Easy to set up, ad-free and reliable comment system
Coment - Online comments aggregated in one dashboard
IntenseDebate - IntenseDebate's comment system enhances and encourages conversation on your blog or website.