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
Realtidbits - Powering the Realtime Social Layer for Entertainment, Local Media and High Traffic Blogs.
HashOver - HashOver is a PHP comment system intended as a replacement for services like Disqus.
JustComments - Easy to set up, ad-free and reliable comment system
utterances - A lightweight comments widget built on GitHub issues.