Jasmin SMS Gateway - Jasmin is a very complete open source SMS Gateway with many enterprise-class features such as:
Pulse SMS - Pulse SMS is a next-generation SMS and MMS application that is fast, secure and packed with lots of modern features as well as customization feature you cloud want.
Kannel - Open Source WAP and SMS Gateway
QKSMS - QKSMS is a beautiful communication app that offers magical texting experience.
Bulk SMS Services - Sending Bulk SMS to the customers will engage the customers with your business and new offers.
Textra - Textra is a seriously beautiful, feature rich SMS and MMS app.