IntenseDebate - IntenseDebate's comment system enhances and encourages conversation on your blog or website.
DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Juvia - A commenting server similar to Disqus and IntenseDebate
RbComment - RbComment - Zend Framework 2 module for drop-in, self-hosted comments, with email notifications...
SolidOpinion - SolidOpinion is a fun, new and dynamic commenting platform.
Commento - A fast, bloat-free comments system to foster discussion on your website