Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.
Sonar - 2-way chat with your customers over SMS and FB Messenger
Crisp Chat - Multi-channel customer support software
ThroughMessage - Uncomplicated customer chat for ecommerce websites
Chatlio - When you have a business to run, your average instant messaging software just will not do. You need an internal presence upon which your entire team can collaborate on. Read more about Chatlio.
Sendblue - Programmatically send iMessage and SMS