Heymarket - The professional way to text message. Your customers are mobile. Heymarket helps you manage those relationships from your smartphone.
Jasmin SMS Gateway - Jasmin is a very complete open source SMS Gateway with many enterprise-class features such as:
SimplyCast - All-in-one platform that allows you to discover, organize, and communicate with your leads over all the channels of communication
Kannel - Open Source WAP and SMS Gateway
Avochato - Text messaging for your business.
Ozeki NG SMS Gateway - Ozeki NG SMS Gateway is a powerful, reliable and flexible SMS Gateway application.