DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Remarkbox - Remarkbox offers a free comment system to keep readers engaged with your content.
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
Thrive Comments - Thrive Comments is a comment system for Wordpress.
Realtidbits - Powering the Realtime Social Layer for Entertainment, Local Media and High Traffic Blogs.