Does anyone knows which 3rd party IM app use the SMS?

Few weeks ago, I got my HTC Desire, an unlock phone. And also change my cell phone plan to T-Mobile Talk+Text (Unlimited Text), without the Data service, but I want to be able to use MSN while I don't have Wi-Fi.

So, is there any app on market that uses SMS, but not uses Data services???

And, what is the setting for network and MMS??