Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.
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.
Avochato - Text messaging for your business.
Crisp Chat - Multi-channel customer support software
HubSpot Conversations - Live chat, bots & team email -- free.
Drift - A messaging app that helps you grow your business.