Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.
Twilio - Brings voice and messaging to your web and mobile applications.
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
MirrorFly - World's Most Scalable Voice, Video & Chat SDK for Small, Medium & Enterprises
JivoChat - JivoChat is a live chat that enables you to talk to visitors on your website
Plivo - Plivo simplifies your customer engagement.