Twilio - Brings voice and messaging to your web and mobile applications.
Jasmin SMS Gateway - Jasmin is a very complete open source SMS Gateway with many enterprise-class features such as:
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Bulk SMS Services - Sending Bulk SMS to the customers will engage the customers with your business and new offers.
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.
MyTask - MyTask is a centralized platform designed for office management, invoice management, and work management that allows a business to have the required service agility.