DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Realtidbits - Powering the Realtime Social Layer for Entertainment, Local Media and High Traffic Blogs.
SolidOpinion - SolidOpinion is a fun, new and dynamic commenting platform.
JustComments - Easy to set up, ad-free and reliable comment system
Commentbox - Commentbox is a jsf component that allows quick realization of a comment functionality
coComment - coComment is a service for managing, powering and researching conversations online.