SendSMS connects to your service provider's web page and pretends to submit a form to their 'Instant Messaging' web page. Currently, Alltel Wireless, AT&T Wireless, Bell Canada/Bell Mobility, Cellular One, Cingular, SkyTel, Sprint PCS, and T-Mobile are supported. Users are encouraged to modify the provided templates to add support for any providers who are currently unsupported.
Sending SMS messages is usually as easy as sending an email; except you send it to the mobile phone service provider formatted however they have it. I.e. <telephonenumber>@messaging.sprintpcs.com works for Sprint PCS and <telephonenumber>@myvzw.com works for Verizon. I've only tried these in the US, YMMV.