Twilio - Brings voice and messaging to your web and mobile applications.
Twinme - Twinme – Private Messenger is a free, secure, instant, and high-definition messaging app that protects your privacy along with giving you full control over each of your relationships and contents.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Textra - Textra is a seriously beautiful, feature rich SMS and MMS app.
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.
Material Messaging - Material Messaging is a beautiful communication app that lets you send and receive SMS and MMS messages using the intuitive interface while giving you complete control over every aspect of your messaging experience.