Internet SMS services are quite common and rather not free.

We build a machine which is able to send error and progress messages by email and optionally by SMS. After our experience SMS providers do change often or even change their login screens so that things change quite a lot and you have to implement different protocols all the time.

Ans SMS terminal (or modem) is quite common I suppose. We decide for this option because this peace of hardware never changes protocol.
You own it and you can send SMS like with any cell phone as long as the SIM card has credits.

There is howver another option which I did not mention, which is an email service.
Some providers offer that you can send an email (in a specific format, containing the SMS and the phone number to send it to) which they pass on as an SMS. This service usually is also not free, but it requires nothing more than an email control like SMPT.ocx or the use of MAPI to send an email.