DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Invision Community - Invision Community is a flexible, integrated set of apps for building communities on the web.
Vuukle - Vuukle is a multi-point user engagement platform.
XenForo - Intuitive. Social. Engaging. Fast. XenForo brings a fresh outlook to forum software.
Commentbox - Commentbox is a jsf component that allows quick realization of a comment functionality
Mobilize - Mobilize is a scalable messaging app built for large groups.