One of the real fun uses of Raspberry Pi is to use it as a FM transmitter. This can be done with very little effort, thanks to a tiny program developed by very creative bunch of guys at the http://blog.codeclub.org.uk/2012/12/08/brief/
http://www.youtube.com/watch?v=blvaYR6aYXA
http://www.icrobotics.co.uk/wiki/index.php/Turning_the_Raspberry_Pi_Into_an_FM_Transmitter
http://www.weedpi.com/how-to/create-an-internet-radio-fm-transmitter/
Now onto Raspberry Pi LF/MF/HF/VHF WSPR Transmitter