DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Reddit - Reddit gives you the best of the internet in one place. Get a constantly updating feed of breaking news, fun stories, pics, memes, and videos just for you.
IntenseDebate - IntenseDebate's comment system enhances and encourages conversation on your blog or website.
Facebook Comments - Facebook comments plugin lets people comment on content on the site using their Facebook account.
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.