Twilio - Brings voice and messaging to your web and mobile applications.
Postscript - Like Mailchimp for SMS
Bandwidth - Bandwidth offers SIP trunking, Emergency communications, and Voice & Messaging APIs.
Tatango - Tatango is a mobile messaging platform for agencies.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Business Texting - Business Texting is an automated business messaging software that will allow your business to engage more customers and boost sales.