DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
JustComments - Easy to set up, ad-free and reliable comment system
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.
utterances - A lightweight comments widget built on GitHub issues.
Facebook Comments - Facebook comments plugin lets people comment on content on the site using their Facebook account.