OpenComment is a Joomla Component thatallows you to accept and moderate comments from the visitors and readers of your site.
They can leave their name, email address and website address as well as rating your article and leaving a comment. It has a lot of Exciting features for a commenting system:

• AJAX, for responsiveness and limiting bandwidth usage
• Rating mechanism and threshold filtering,
• Comments sorting and searching
• User alert,
• Anti spamming plugins support, automoderation,
• Smilies pack themes
• More than 60 settings in admin panel
• Fully i18N
• build with security in mind
• GNU/GPL licence!