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
IntenseDebate - IntenseDebate's comment system enhances and encourages conversation on your blog or website.
HashOver - HashOver is a PHP comment system intended as a replacement for services like Disqus.
Vuukle - Vuukle is a multi-point user engagement platform.
utterances - A lightweight comments widget built on GitHub issues.