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:
KAPSYSTEM - KAP Computer Solutions Pvt. Ltd.,
Ozeki NG SMS Gateway - Ozeki NG SMS Gateway is a powerful, reliable and flexible SMS Gateway application.
Kannel - Open Source WAP and SMS Gateway
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates