Jasmin SMS Gateway - Jasmin is a very complete open source SMS Gateway with many enterprise-class features such as:
Octopush - Octopush is a company providing technology-based communication solutions, including SMS, Web solutions, and Email APIs.
Kannel - Open Source WAP and SMS Gateway
E-goi - Email Marketing companies, SMS Marketing, software email marketing.
Ozeki NG SMS Gateway - Ozeki NG SMS Gateway is a powerful, reliable and flexible SMS Gateway application.
CheapGlobalSMS - Send instant bulk SMS worldwide, with reliable SMS delivery and reports in real-time.