DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Stack Overflow - Community-based Q&A part of the Stack Exchange platform.
Hyvor Talk - The Best Commenting Platform For Your Website
The Answer Bank - The Answer Bank is the UK’s leading question-and-answer site.
Commento - A fast, bloat-free comments system to foster discussion on your website
Blurtit - Blurtit is a Q&A website. You can ask a question or answer on any question.