DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
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
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.
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.
utterances - A lightweight comments widget built on GitHub issues.
idea.informer - Idea Informer is a idea forum similar to uservoice or getsatisfaction.