PiFM and RPiTX - two projects to turn your Raspberry Pi into an FM Transmitter
The subject for this post speaks for itself!
PiFM - http://www.icrobotics.co.uk/wiki/index.php/Turning_the_Raspberry_Pi_Into_an_FM_Transmitter
RPiTX - https://github.com/F5OEO/rpitx
Be sure to build filters if you want to transmit on ham bands as the output WILL splatter.
Joe de VA3POR