Free SMS API (Zeep)

Zeep Mobile is a simple ad-supported SMS API that any web app can use to communicate with its users via SMS for free. To use the Zeep Mobile API, you'll need to generate an API and Secret Key for each website you want to use the API for. These keys are used to authenticate requests to and from Zeep Mobile. You can also use this API to receive SMS messages. Messages are posted to your callback url whenever your users interact with Zeep via their phone or our settings panel. The Content-type of the POST is application/x-www-form-urlencoded. This means you can treat it almost exactly as if it were a form post from a web page. The only difference you'll notice is in how you respond.
More information: http://www.zeepmobile.com/