Jasmin SMS Gateway - Jasmin is a very complete open source SMS Gateway with many enterprise-class features such as:
Dingtone - Dingtone is a free call and texting application for making unlimited calls and text messages to any number at the moment registered in any part of the world by any cellular company.
Ozeki NG SMS Gateway - Ozeki NG SMS Gateway is a powerful, reliable and flexible SMS Gateway application.
Textnow.com - It lets you text your friends, aka my only source of happiness
Bulk SMS Services - Sending Bulk SMS to the customers will engage the customers with your business and new offers.
FluffyChat - Open. Nonprofit. Cute ?