Tropo - Tropo is a cloud API for voice and messaging.
Twilio - Brings voice and messaging to your web and mobile applications.
Melrose Labs SMSC Simulator - SMPP SMSC simulator to test SMS messaging applications.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
SMSAPI - SMSAPI.com is browser platform for bulk SMS and other SMS marketing solutions.
Plivo - Plivo simplifies your customer engagement.