Releans - Communication platform for Messaging and Chatbots
Twilio - Brings voice and messaging to your web and mobile applications.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
BulkSMS - SMS messaging service provider offering two-way SMS communication services.
BudgetSMS.net - Cheap SMS Gateway with simple HTTP API to send SMS messages worldwide.
Plivo - Plivo simplifies your customer engagement.