Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
HiOperator - HiOperator is a virtual assistant that answers phone calls, chats with customers, provides in-app help, takes orders, and provides support.
Kayako - Help Desk Software and Customer Service Software by Kayako. Improve customer satisfaction, increase sales.
Service - Customer service issues solved for you, on demand, for free.
Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.
HelpStack - Open Source mobile help framework to support iOS and Android app users.