This may be a bit beyond many broadcasters here technically, but it sure is cool. The Raspberry Pi, that tiny little computer for hobbyists and tinkers is capable of generating an FM signal. It's a hack and the software is a bit limited so far.
Total output on this is estimated at 14mw.
http://www.icrobotics.co.uk/wiki/index.php/Turning_the_Raspberry_Pi_Into...
That solution is limited to a mono broadcast.
Someone outdid them and created a stereo version :)
http://www.reddit.com/r/raspberry_pi/comments/14k5o3/raspberry_pi_fm_tra...
There are multiple videos on YouTube about this:
http://youtu.be/blvaYR6aYXA