DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
JLex Comment - JLex Comment is a comment system for Joomla.
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Commentics - Commentics is an advanced PHP comment script with many features.
Commentbox - Commentbox is a jsf component that allows quick realization of a comment functionality
Remarkbox - Remarkbox offers a free comment system to keep readers engaged with your content.