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.
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
P3chat.com - P3chat is an innovative and easy Live Chat & Support service.