SendBird - SendBird is messaging-as-a-service.

EssentialPIM - EssentialPIM is a free Personal Information Manager that keeps up with the times and lets you manage appointments, tasks, notes, contacts, password entries and email messages across multiple devices and cloud applications.

Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.

Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.

Live800 - Live800 is an enterprise-grade Live Chat and Customer Service platform that enables businesses to provide instant support to their customers through a variety of channels, including Live Chat, Email, Phone, and Social Media.

Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.