hey secret4all
I am looking for the same thing, only problem I have is I want it for just the local company that services my area, and the ones mentioned previously all require logins or payment by the users, let alone some I have seen outside of the forum wanting to charge astronomical figures to license the technology on your web site.
The one dumbo mentions seems it would be a good value, but for me I cannot hook up a GSM phone to my web host as they are about 800 miles away, and most won't allow that any way, unless they already have the capabilities.
The other thing is depending upon the service the phone user is on, sometimes you can just send an email to
phonenumber@theirprovider.com The problem exists that not all mobile telecom companies support this, and also the fact that many have their own proprietary sms gateways. One page had about 150 different PHP scripts for varous communications companies to interface with their system for message sending! Whew!
Maybe post and let us know what market you are serving (just a local market, regional, internation etc) and if you want to send to any mobile phone or just local area. And definitely keep us posted on progress, and I will do the same.