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

UserTesting.com - Usability testing has never been easier. Get videos of real people speaking their thoughts as they use websites, mobile apps, prototypes and more!

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.

Userbrain - Easy, fast, and affordable user testing for websites and prototypes.

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

UserTest.io - User testing made simple. High-quality, affordable user testing solutions from real users, here in the UK. Improve your usability!