Twilio - Brings voice and messaging to your web and mobile applications.
SlickText - Slick Text provides businesses and organizations with an easy and affordable platform for text message marketing.
Bandwidth - Bandwidth offers SIP trunking, Emergency communications, and Voice & Messaging APIs.
SimpleTexting - SimpleTexting offers SMS and text marketing services for companies of all sizes.
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Textmagic - The all-in-one solution for all your business texting needs - marketing campaigns, two-way SMS chats, reminders, notifications, and internal staff communication.