DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.

IntenseDebate - IntenseDebate's comment system enhances and encourages conversation on your blog or website.

Discourse - Discourse is an open source discussion platform built for the next decade of the Internet.

Viafoura - Viafoura is an easy-to-integrate social monetization platform that enables your to better engage, understand and monetize your community.

Thrive Comments - Thrive Comments is a comment system for Wordpress.

GraphComment - Add modern discussion and social interactions to your website