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.
Live Chat by Sumo.com - Instantly communicate with customers, prospects & visitors
Chat Center - Chat with one click, no matter what messaging app you use
Drift - A messaging app that helps you grow your business.
Helpshift - Helpshift is an in-game customer support platform that enables mobile apps to improve customer experience, drive higher ratings and increase retention.