Jasmin SMS Gateway - Jasmin is a very complete open source SMS Gateway with many enterprise-class features such as:
Twilio - Brings voice and messaging to your web and mobile applications.
Kannel - Open Source WAP and SMS Gateway
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Bulk SMS Services - Sending Bulk SMS to the customers will engage the customers with your business and new offers.
BulkSMS - SMS messaging service provider offering two-way SMS communication services.