DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Optimizely - A/B testing you'll actually use.
Reddit - Reddit gives you the best of the internet in one place. Get a constantly updating feed of breaking news, fun stories, pics, memes, and videos just for you.
VWO - Use VWO to tweak, optimize & personalize your website with minimal IT help.
Facebook Comments - Facebook comments plugin lets people comment on content on the site using their Facebook account.
Google Analytics - Improve your website to increase conversions, improve the user experience, and make more money using Google Analytics. Measure, understand and quantify engagement on your site with customized and in-depth reports.