DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Remarkbox - Remarkbox offers a free comment system to keep readers engaged with your content.
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
talkatv - talkatv is an open source comment service much like ...
Realtidbits - Powering the Realtime Social Layer for Entertainment, Local Media and High Traffic Blogs.