UseResponse - Open-source, self-hosted customer feedback software, live chat and helpdesk system that you can install on your server. Organize documentation using knowledge base and get feedback from social networks with centralized system
DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
UserVoice - UserVoice integrates easy-to-use feedback, helpdesk, and knowledge base management tools in one platform that empowers users to speak and companies to understand.
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.
Survicate - Collect feedback on your website and find out more about your visitors.
utterances - A lightweight comments widget built on GitHub issues.