Twilio - Brings voice and messaging to your web and mobile applications.
Jasmin SMS Gateway - Jasmin is a very complete open source SMS Gateway with many enterprise-class features such as:
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.
Kannel - Open Source WAP and SMS Gateway
Melrose Labs SMSC Simulator - SMPP SMSC simulator to test SMS messaging applications.
Bulk SMS Services - Sending Bulk SMS to the customers will engage the customers with your business and new offers.