DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Remarkbox - Remarkbox offers a free comment system to keep readers engaged with your content.
Answerbase - Add a Q&A system to your website in just minutes, with Answerbase's powerful question and answer software for online communities and customer support.
Commento - A fast, bloat-free comments system to foster discussion on your website
utterances - A lightweight comments widget built on GitHub issues.
Realtidbits - Powering the Realtime Social Layer for Entertainment, Local Media and High Traffic Blogs.