Jasmin SMS Gateway - Jasmin is a very complete open source SMS Gateway with many enterprise-class features such as:
Kannel - Open Source WAP and SMS Gateway
KAPSYSTEM - KAP Computer Solutions Pvt. Ltd.,
Power SMPP - Enterprise grade messaging software
BudgetSMS.net - Cheap SMS Gateway with simple HTTP API to send SMS messages worldwide.
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.