Twilio - Brings voice and messaging to your web and mobile applications.
TapTalk.io - TapTalk.io is a complete in-app chat SDK and messaging API - Easy integration and customizeable.
Bandwidth - Bandwidth offers SIP trunking, Emergency communications, and Voice & Messaging APIs.
Lets Chat - Open source self-hosted chat app for small teams
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
WeChat API - WeChat API and SDK allow developers to access and integrate the functionality of WeChat with other applications.