Jasmin SMS Gateway - Jasmin is a very complete open source SMS Gateway with many enterprise-class features such as:
Fastsms - Fastsms helps you with marketing by providing business text messaging which you can use to send information regarding discounts and delivery details to customers.
Kannel - Open Source WAP and SMS Gateway
Salesmsg - Salesmsg is a simple tool that gives businesses an easy way to text their contacts online from local phone numbers.
Ozeki NG SMS Gateway - Ozeki NG SMS Gateway is a powerful, reliable and flexible SMS Gateway application.
MyTask - MyTask is a centralized platform designed for office management, invoice management, and work management that allows a business to have the required service agility.