DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
talkatv - talkatv is an open source comment service much like ...
coComment - coComment is a service for managing, powering and researching conversations online.
Commentics - Commentics is an advanced PHP comment script with many features.
komentify - Enrich your website with comments in less than 30 seconds
Commento - A fast, bloat-free comments system to foster discussion on your website