Can you guys suggest me any good FREE product review system? The functionality should include commenting on the reviews by customers, Something like 'Disqus'.
I don't know of any free ones, however this appears to be a decently priced extension at 9.99EU: http://shop.tentura.eu/magento-extensions/disqus-for-magento.html
It implements Disqus into Magento, providing the exact functionality you're looking for.
Also, it appears they've released a "light" version of this plugin for free as well: https://github.com/involic/Magento-DISQUS-Comments-Integration-Light
Seems a little out dated, but may still work.
Yotpo is a startup that seems to be picking up some steam - they've been at a few Magento conferences.
Having demoed their platform I think it provides the functionality you're looking for. The plugin is free; the SaaS solution is free up to a certain number of visitors monthly. So this may cost you money in the end if you have a large amount of traffic.