DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Master Comments System - Master Comments System is a very compact comments system with basic options:
Commento - A fast, bloat-free comments system to foster discussion on your website
utterances - A lightweight comments widget built on GitHub issues.
talkatv - talkatv is an open source comment service much like ...
JustComments - Easy to set up, ad-free and reliable comment system