Theres a thread here called Free SMS that uses a new feature AIM is now offering to mobiles. It will allow people to send you a TXT to a phone and carry on the conversation in AIM, however to the other person, they see it as normal TXT. So basically you are using your unlimited internet minutes to send aim-IMs as TXTs to other people, for FREE.
It stops working once you're no longer on AIM though, and you have to add the people you want to IM on to your contact list from a desktop computer which is a bummer.
Do a search and find the thread, im sorta lazy right now.