SendBird - SendBird is messaging-as-a-service.
uhChat - uhChat is a chat window software that can be integrated into any website.
Chat by Stream - Build and ship in app messaging in a few hours
Chatroll - Chatroll is the leading provider of interactive tools for social publishers.
Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.
tawk.to - tawk.to is a free live chat app that lets you monitor and chat with visitors on your website or from a free customizable page