As a small sender, you have a couple of options.
1. Interface with an SMS Gateway
Companies like Clickatell
and others (just google for "sms gateway" and you'll find plenty) generally provide you with a submission form or API that you can use to send text messages to just about any provider.
Depending on your volume and the destination, most messages cost from 5 to 10 cents (US) each
2. Use email-to-SMS gateways provided by carriers.
This generally only applies to US/Canada carriers, as they charge for both sending and receiving SMS.
In short, you send a message to a special <phone number>@carrier.tld
e-mail address, which is converted into a text message and sent to the recipient.
This is as free for you as sending an e-mail.
Wikipedia has a table of Email-to-SMS gateway addresses
Note that most larger senders that require reliable delivery are either eating the cost for free SMS (paying for and going through a gateway), or if they're large enough, interfacing with carriers directly.