Twilio - Brings voice and messaging to your web and mobile applications.
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.
TelAPI - Build Apps With Our Cloud Based SMS & Voice API
SMSAPI - SMSAPI.com is browser platform for bulk SMS and other SMS marketing solutions.
Crypton.sh - SMS - Secure Anonymous SMS in the cloud (and a phone number)
Tellephant - Helping you build incredible chatbots and integrate WhatsApp, Google, Facebook, SMS and more to your communication stack in seconds. All at the cheapest rates out there.