DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Schnack - Schnack is an open source commenting system written in JavaScript.
Answerbase - Add a Q&A system to your website in just minutes, with Answerbase's powerful question and answer software for online communities and customer support.
GitHub Team Discussions - Now there's more space to talk through your projects 💬
utterances - A lightweight comments widget built on GitHub issues.
Product Hunt Reviews Widget - Embed and encourage PH reviews into your product