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.
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.
talkatv - talkatv is an open source comment service much like ...
utterances - A lightweight comments widget built on GitHub issues.
Commento - A fast, bloat-free comments system to foster discussion on your website