Jasmin SMS Gateway - Jasmin is a very complete open source SMS Gateway with many enterprise-class features such as:
LINK Mobility - LINK Mobility is Europe's leading provider within mobile communications, specialising in mobile SMS messaging, digital services and multi-channel conversational communication.
Kannel - Open Source WAP and SMS Gateway
Melrose Labs SMSC Simulator - SMPP SMSC simulator to test SMS messaging applications.
Ozeki NG SMS Gateway - Ozeki NG SMS Gateway is a powerful, reliable and flexible SMS Gateway application.
Clockwork SMS - Clockwork is an Easy Text Message API. An SMS API for Developers. Build powerful apps and include SMS. Signup is free, Try it now.