Yeah I have one I even paid for the premium version, called textfree. It has its advantages and disadvantages.
Advantages: It runs on data and not SMS. Delivery time is really good, always instant from what I've seen.
Disadvantages: The iphone doesn't have push notification until OS 3.0 hits in a few days so I have to check for new texts via email and the shortest time is 15 min intervals. The number shown to the recipients is actually am email wich on most phones you can't enter as a contact, or at least we haven't figured it out yet. Lastly every once in a while a text to me will bounce. So it's not a total solution.
Push notification will help a lot though.