Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Twilio - Brings voice and messaging to your web and mobile applications.
46elks - Adding SMS and voice to your application or service should be easy.
Atomic SMS Sender - Free bulk SMS software to send bulk text messages from PC
freepbx - FreePBX is a web-based open source GUI that controls and manages Asterisk (PBX).
Clickatell - Simple, reliable mobile messaging cloud.