DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Commento - A fast, bloat-free comments system to foster discussion on your website
Hyvor Talk - The Best Commenting Platform For Your Website
utterances - A lightweight comments widget built on GitHub issues.
Cusdis - Open-source, lightweight comment system for your website
JustComments - Easy to set up, ad-free and reliable comment system