DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
vBulletin - Build a community website with the leading forum software, vBulletin 5 Connect, or in the cloud on vB Cloud!
Vuukle - Vuukle is a multi-point user engagement platform.
Discourse - Discourse is an open source discussion platform built for the next decade of the Internet.
Commentbox - Commentbox is a jsf component that allows quick realization of a comment functionality
XenForo - Intuitive. Social. Engaging. Fast. XenForo brings a fresh outlook to forum software.