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
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.
Doorbell.io - Collect in-app user feedback. Available on websites, iOS, and Android.
idea.informer - Idea Informer is a idea forum similar to uservoice or getsatisfaction.
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.
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.