The EAPI (HTTP to SMS API) is a RESTful API that uses simple query parameters via URLs. If you can put together a Web page with a form, backed by a scripting language (e.g. PHP), you already have most of the skills that you need to send SMSs.
To get started, read the EAPI manual - or sign up and jump straight to Code Samples - have something up and running in minutes without needing to buy anything.