DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Epoch by Postmatic - A native WordPress Disqus alternative with a focus on speed and privacy: 100% realtime native...
Commento - A fast, bloat-free comments system to foster discussion on your website
utterances - A lightweight comments widget built on GitHub issues.
Joint Comments - An embeddable comment widget for websites, blogs, ecommerce & static pages.
HTML Comment Box - HTML Comment Box widget for your website. Copy the HTML code and it just works!