DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
JustComments - Easy to set up, ad-free and reliable comment system
coComment - coComment is a service for managing, powering and researching conversations online.
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
Facebook Comments - Facebook comments plugin lets people comment on content on the site using their Facebook account.