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.

KISSinsights - Give customers a way to tell you what they need. Get real-time feedback from your customers on your website, free.

Zendesk - Zendesk is a beautiful, lightweight help-desk solution.

Survicate - Collect feedback on your website and find out more about your visitors.

Freshdesk - Freshdesk is a cloud-based customer support software that lets you support customers through traditional channels like phone and email, social channels like Facebook and Twitter, and your own branded community