Add SMS functionnalityto existing Web Site

Hi, the title says it all. We have a web site on [login to view URL] (web server). We need to add the SMS functionality to it (just sending, not receiving). I need that without having to use a GSM modem. The best would be to use a free online provider or a very cheap way to do it with SMS Gateway or??? Something like a free online service or a very small fee paid by the phone owner (like 0.03 $ per SMS) or a very cheap monthly fee subscription (like 5$) to get a lot of SMS messages... Need to get this done very fast. Thank you