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.
DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
GetSatisfaction - Get Satisfaction is the leading customer engagement platform that helps companies build better relationships with their customers and prospects, through the best online customer community.
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.
idea.informer - Idea Informer is a idea forum similar to uservoice or getsatisfaction.
utterances - A lightweight comments widget built on GitHub issues.