Have you ever heard about TextFreek? Probably no.
TextFreek is a free App that allows you to send text messages between devices that share this App. This messages are completely free, infact you just have an 3G connection or WiFi connection. The fairness of this App is that you can sent free text messages between iPhone but also Android and BlackBerry.
Infact the App's developer has released the same App for iPhone, BlackBerry and Android.
So pick up your devices and search your app:
iPhone -> TextFreek on AppStore
Android -> ChompSMS on Android Market
BlackBerry -> CrunchSMS on AppWorld
The App is free for both devices, here the Official TextFreek Web page.
So, Start texting!! :)