As IconOne said you can use AOL's IM, the app is free on the App store. My friends will send me a regular text to tell me to start AOL' IM and then we go for hours. It uses the internet so it's unlimited.
or you can also get an aim account, or yahoo, or msn or w/e u choose and text the person u wish to choose through that...
for example if you wanted to text through aim you would type +1 area code then the number
only prob is that u have to keep the aim app on in order to receive the text from others