Clickatell[1] is cheap and provides a number of methods to send SMS: email-to-sms, HTTP (XML, SOAP, urls), SMPP, FTP. They also support inbound/two-way messaging and shortcodes, though I assume these cost a good bit more.<p>[1] <a href="http://www.clickatell.com/" rel="nofollow">http://www.clickatell.com/</a>