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.
Twilio - Brings voice and messaging to your web and mobile applications.
Plivo - Plivo simplifies your customer engagement.
smooch - Smooch connects your business software to all the world’s messaging channels for a more human customer experience.
SMSAPI - SMSAPI.com is browser platform for bulk SMS and other SMS marketing solutions.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates