computer?

Somewhere on their site, but last time I checked, they bury it pretty well. Try: Text Message - Send Free Text Messages on the web!

Their website allows you to send a text message to someone on the Cingular network. The link is below:

www.cingular.com

You can also send an email to 10-digit-phone-number@cingularme.com (as in 1234567890@cingularme.com) and it will be sent as a text message to the owner of the phone number, provided they are a Cingular subscriber. This works with prepaid or contract Cingular and also with Tracfones that are on the Cingular coverage area.

That actually doesn't work anymore. The only place it can be done now is from the cingularme.com website and you have to create a log in if you do not have one. Cingular implemented this to force more people to send messages phone to phone rather than from the website.

Dang... talk about complicating things...

cingular.com works, you have to log into your account, then hit GO on phone/device, then at the bottom right, you can send texts

above that place, it says:
Text messages can only be sent to other Cingular subscribers. Recipients will be charged according to their Text Messaging service plan.

Send the text message *as an email* from your own email provider. Send it to xxxxxxxxxx@cingularme.com where you substitute the x's for the recipient's phone number.

QUESTION:
Why can't I find the option to send a text message from Cingular.com?

ANSWER:
Users of My Account can send text messages to other Cingular customers by following these instructions:

Login to My Account and click on the Go button in the in the Phone/Device box or click here: Phones & Devices (logged in users only).
Scroll down the page to the Send a Text Message box.
Enter the Cingular Wireless recipient's phone number in the To: field. Only one phone number may be entered at a time. You don't have to enter dashes as they will be automatically entered for you.
Enter a message in the Message: field. There is a limit of 150 characters.
Click on Send.
If the message was sent successfully to a Cingular customer, you will see the following message at the top of the screen: Your message has been submitted for delivery. Standard messaging rates apply for the recipient.

The @cingularme.com method works best.

That's my method of choice also.