Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
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
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.
Heeduser - Heeduser is the customer engagement platform that helps companies build better relationships with their customers and prospects, through the better online customer community
P3chat.com - P3chat is an innovative and easy Live Chat & Support service.