DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Joint Comments - An embeddable comment widget for websites, blogs, ecommerce & static pages.
LiveFyre - livefyre-subscriptions - A isomorphic JavaScript library for managing Subscriptions in Livefyre's Personalized News Stream service.
Hyvor Talk - The Best Commenting Platform For Your Website
Muut - The complete discussion system for your site. Forums, commenting and messaging right on your site. Unlimited posts, users and traffic, for free.
Widget Pack Comment System - Its comes totally free for one website. Using it is very easy too.