Twilio - Brings voice and messaging to your web and mobile applications.
Avochato - Text messaging for your business.
Bandwidth - Bandwidth offers SIP trunking, Emergency communications, and Voice & Messaging APIs.
Postscript - Like Mailchimp for SMS
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
GetChat.App - Add WhatsApp chat to your website for free