Some great work from the team at IC Robotics has turned the Pi into an FM radio! Capable of playing back wav files via a Python application (backed with a custom C library).
At the moment it’s Mono only and there are some issues with CPU priority but it’s a great starting point.
And there’s more, Guido PE1NNZ has released code to enable the Raspberry Pi computer board to generate SSB on the 7 and 14 MHz bands. SSB is used in HAM radio around the world and Guido has managed to make contact with other users across Europe via his Pi.
Disclaimer: producing radio signals may be subject to control and licensing in your country. Please check your local laws before trying any of the above.