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
Ozeki NG SMS Gateway - Ozeki NG SMS Gateway is a powerful, reliable and flexible SMS Gateway application.
Skype - Stay in touch with your family and friends for free on Skype. Download Skype today to chat and call on desktop and mobile.
Bulk SMS Services - Sending Bulk SMS to the customers will engage the customers with your business and new offers.