Epoch by Postmatic - A native WordPress Disqus alternative with a focus on speed and privacy: 100% realtime native...
Juvia - A commenting server similar to Disqus and IntenseDebate
DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Joint Comments - An embeddable comment widget for websites, blogs, ecommerce & static pages.
Realtidbits - Powering the Realtime Social Layer for Entertainment, Local Media and High Traffic Blogs.
Muut - The complete discussion system for your site. Forums, commenting and messaging right on your site. Unlimited posts, users and traffic, for free.