This site shows you how to use a $35 Raspberry Pi Computer as an RF transmitter for the WSPR (Weak Signal Propagation Reporting) System. All you need is a simple low pass filter and an antenna. (Oh yea, and a ham radio license.) The site says you can get 10 milliwatts out. That's enough for WSPR! Very cool.
This looks like a real international effort:
Credits goes to Oliver Mattos and Oskar Weigl who implemented PiFM  based on the idea of exploiting RPi DPLL as FM transmitter. Dan MD1CLV combined this effort with WSPR encoding algorithm from F8CHK, resulting in WsprryPi a WSPR beacon for LF and MF bands. Guido PE1NNZ extended this effort with DMA based PWM modulation of fractional divider that was part of PiFM, allowing to operate the WSPR beacon also on HF and VHF bands.
For more info on WSPR: http://soldersmoke.blogspot.com/search/label/WSPR
Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20