Yep,
Tango Messenger works very nicely on iOS, Android, Windows 7, Windows Phone, and Blackberry. But STILL not on OSX!
I use it on my Samsung Galaxy Note (Android) all the time, but I hate not being able to run it on my Macs!
Tango applies (free) instant messaging (via internet) if it can, and (expensive) mobile networks when it must.
Just one more gripe: I can't attach files/photos to a message. They have to be sent as a separate message.