Jasmin SMS Gateway - Jasmin is a very complete open source SMS Gateway with many enterprise-class features such as:
Ozeki NG SMS Gateway - Ozeki NG SMS Gateway is a powerful, reliable and flexible SMS Gateway application.
Kannel - Open Source WAP and SMS Gateway
KAPSYSTEM - KAP Computer Solutions Pvt. Ltd.,
BudgetSMS.net - Cheap SMS Gateway with simple HTTP API to send SMS messages worldwide.
SuperPhone - SuperPhone serves as an intelligent inbox for SMS that filters, organizes, segments, and automates messages over SMS and Facebook Messenger.