Many cell (mobile) phone providers in the USA have an email gateway for SMS that allows anyone to send an email that will in turn send a text message. For example, to send a
You might also like to consider the skype route which has the advantage that you can use it how and when you like and its completey under your control - email to sms But I have to admit that I use a third party email to sms gateway and it works fine but its has limitations on what email addresses can be converted to sms i.e. they have to be sent to the companies SMTP server using a supplied email address
The email-to-SMS gateways are generally reliable, given a few conditions (read below). Whether you should use an email-to-SMS gateway or an actual third-party provider depends on the application. If you're going to need a lot of control over the formatting of the messages, collecting replies, etc., then go with a third-party provider. If it's a simple notification you are sending your users, go with email-to-SMS.
If you send too many messages from the same email address, the messages will start backing up. For instance, I sent a message out to about 3,000 AT&T Wireless recipients, and some received the messages up to 3 days later. But typically, sending 10-30/minute won't cause any problems.
I use email-to-SMS gateways for an iPhone application that I built. It sends probably 10 messages per minute to almost all U.S. carriers, and it rarely experiences problems.
There are three main issues that I run into with the email-to-SMS gateway: