App to send SMS messages from mobile phone using data connection?

[Jameslong]

I'm looking for an application which will run on a mobile phone (presumably Java based) which will allow people to compose SMS messages and then to send these using the phone's internet connection instead of the usual telecoms company SMS gateway. The app would use a protocol such as HTTP + webservices to talk to a web server. Does anything like this exist? (I am in the UK, not sure if this is relevant)

thanks