Melrose Labs SMSC Simulator - SMPP SMSC simulator to test SMS messaging applications.
Twilio - Brings voice and messaging to your web and mobile applications.
Tropo - Tropo is a cloud API for voice and messaging.
Plivo - Plivo simplifies your customer engagement.
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.
MessageBird - Reach 7 billion phones in seconds via SMS, Chat & Voice. Try it for free and improve your communication.