Slack - A messaging app for teams who see through the Earth!
Twilio - Brings voice and messaging to your web and mobile applications.
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Mattermost - Mattermost is an open source alternative to Slack.
Clockwork SMS - Clockwork is an Easy Text Message API. An SMS API for Developers. Build powerful apps and include SMS. Signup is free, Try it now.