Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Jasmin SMS Gateway - Jasmin is a very complete open source SMS Gateway with many enterprise-class features such as:
Twilio - Brings voice and messaging to your web and mobile applications.
Kannel - Open Source WAP and SMS Gateway
Atomic SMS Sender - Free bulk SMS software to send bulk text messages from PC
Bulk SMS Services - Sending Bulk SMS to the customers will engage the customers with your business and new offers.