Joe Desbonnet has made what can only described as the most inexpensive way of interfacing an Arduino board with an Android phone bar yelling at it and hoping for a response. In his project, he uses a meter of enamelled copper wire, a resistor and a diode to send data at a speed that makes you wish for a telex machine.
In all seriousness, Joe's project is certainly novel and out of the box - the data is sent by creating a low-frequency signal through the coil which is them detected by the magnetometer in the Android phone and measured with a simple app. You can see it in action here:
If you have an Android phone or similar hardware, this is a simple and expensive experiment to try out for fun. If you want to make your own rendition - you could build the external circuit and coil onto a protoshield, such as our inexpensive ProtoShield Basic for Arduino:
In the meanwhile, Joe has made all the instructions, code and notes available on his site here.