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
Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
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
Olark - Real-time chat and monitoring of your website visitors through Google Talk and Jabber.
Woopra - Track your customers' web and mobile activity, forms, emails, support tickets and more, all in one place with customer analytics. Analyze and take action.