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
Master Comments System - Master Comments System is a very compact comments system with basic options:
HashOver - HashOver is a PHP comment system intended as a replacement for services like Disqus.
utterances - A lightweight comments widget built on GitHub issues.
Juvia - A commenting server similar to Disqus and IntenseDebate