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.

Community Questions for Confluence - Keep questions and answers in one place with an engaging, community-driven Q&A discussion forum, powered by Confluence

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

Question2Answer - A Q&A site helps an online community to share knowledge.

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.

Stack Overflow for Teams - Everything you love about Stack Overflow in a private space.