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
Feedbackify - Feedbackify offers website feedback solutions that enable companies to engage with their customers.
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.
tawk.to - tawk.to is a free live chat app that lets you monitor and chat with visitors on your website or from a free customizable page
Survicate - Collect feedback on your website and find out more about your visitors.
Userlike - Userlike is free live chat software for website and mobile support.