Slack - A messaging app for teams who see through the Earth!
TapTalk.io - TapTalk.io is a complete in-app chat SDK and messaging API - Easy integration and customizeable.
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Lets Chat - Open source self-hosted chat app for small teams
Mattermost - Mattermost is an open source alternative to Slack.
WeChat API - WeChat API and SDK allow developers to access and integrate the functionality of WeChat with other applications.