Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
Feedbackify - Feedbackify offers website feedback solutions that enable companies to engage with their customers.
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
Userlike - Userlike is free live chat software for website and mobile support.
P3chat.com - P3chat is an innovative and easy Live Chat & Support service.
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