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
HashOver - HashOver is a PHP comment system intended as a replacement for services like Disqus.
utterances - A lightweight comments widget built on GitHub issues.
Cusdis - Open-source, lightweight comment system for your website
Commentics - Commentics is an advanced PHP comment script with many features.