Twilio - Brings voice and messaging to your web and mobile applications.

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

Bandwidth - Bandwidth offers SIP trunking, Emergency communications, and Voice & Messaging APIs.

JivoChat - JivoChat is a live chat that enables you to talk to visitors on your website

Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates

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