If it's just one person you'd like to text message for free (free for you, what ever it costs her)

You can use the "email to sms gateways" all the providers have.

http://en.wikipedia.org/wiki/SMS_gateway

Her phone number of course is: 212-555-1212 and she's a smart girl and uses t-mobile (why pay for the same minutes right?)

So you simply email: 2125551212@tmomail.net and she'll get it. And most providers allow her to reply to you and you'll actually receive it in your email.

(at least I can receive replies in my gmail, but for sure ANY email provider will let you SEND to them.)

At the top of the wikipedia page there's also a way they can start the email conversion ( like in the t-mobile example having her text 500 with the message in the format: emailaddress #subject# text ) then you'd receive it.