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.
JLex Comment - JLex Comment is a comment system for Joomla.
Commento - A fast, bloat-free comments system to foster discussion on your website
Commentics - Commentics is an advanced PHP comment script with many features.
Realtidbits - Powering the Realtime Social Layer for Entertainment, Local Media and High Traffic Blogs.