DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
SolidOpinion - SolidOpinion is a fun, new and dynamic commenting platform.
Commento - A fast, bloat-free comments system to foster discussion on your website
Commentbox - Commentbox is a jsf component that allows quick realization of a comment functionality
FastComments - A very fast, feature-full, and privacy-focused comment service for handling discussions on the internet. At 4.6 kB with no dependencies, it's the fastest comment service around.
IntenseDebate - IntenseDebate's comment system enhances and encourages conversation on your blog or website.