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