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 - Community-based Q&A part of the Stack Exchange platform.
Survicate - Collect feedback on your website and find out more about your visitors.
The Answer Bank - The Answer Bank is the UK’s leading question-and-answer site.