Jasmin SMS Gateway - Jasmin is a very complete open source SMS Gateway with many enterprise-class features such as:
Targlets.com - A kind SMS platform. Bulk SMS, Marketing and API.
Kannel - Open Source WAP and SMS Gateway
FrontlineSMS - FrontlineSMS lowers barriers to social change using mobile technologies.
Ozeki NG SMS Gateway - Ozeki NG SMS Gateway is a powerful, reliable and flexible SMS Gateway application.
KAPSYSTEM - KAP Computer Solutions Pvt. Ltd.,