TextBelt
A free, open source API for outgoing texts.
Maintained by Ian Webster. Open source on github.
TextBelt is a REST API that sends outgoing SMS. It is available as a public service or as an internally self-hosted server.
Textbelt uses carrier-specific gateways to deliver your text messages for free, and without ads. The service is fairly reliable and has sent over 1 million texts.
Send a text with a simple POST request: