Guido PE1NNZ has released code that he says enables the Raspberry Pi to operate as a WSPR transmitter at HF and VHF.
Guido has modified the WsprryPi code originally developed by Dan Ankers MD1CLV. He says the Raspberry Pi requires just a Low Pass Filter (LPF) and antenna connected to the GPIO4 pin in order to function as a WSPR transmitter.
The PE1NNZ code and binary - https://github.com/threeme3/WsprryPi
Dan Ankers MD1CLV WSPR - http://m0xpd.blogspot.co.uk/2013/01/raspberry-whispers.html
WSPR on the Raspberry Pi - http://www.southgatearc.org/news/march2013/wspr_on_the_raspberry_pi.htm