I have been look for a free SMS API since April, when my previous free SMS provider, IMified, announced its shutdown.
Zeep Mobile claims to be free forever (because it is ad-supported), but my sign-up is unanswered after a few weeks. Twilio and Tropo both give developers some free credit, but they charge money for production use. Other resources are suggesting an in-house solution: buy a SIM card with cheap / unlimited text plan, plug it into an old phone, and connect to the server; this still costs money, and is impossible for those using a VPS.
As a Google Voice user, I'm thinking: can I use Google Voice to build a free SMS robot?
Google Voice API
Google, the company who release an API for almost every product, does not provide a Google Voice API.