DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
HashOver - HashOver is a PHP comment system intended as a replacement for services like Disqus.
Commento - A fast, bloat-free comments system to foster discussion on your website
talkatv - talkatv is an open source comment service much like ...
utterances - A lightweight comments widget built on GitHub issues.
Joint Comments - An embeddable comment widget for websites, blogs, ecommerce & static pages.