I just listened to a "Stuff You Should Know" podcast on Numbers Stations. These are shortwave radio transmissions of seemingly random number or letters (or both). They are thought to be used for spying purposes, and were thought to have been used right after WWII and during the Cold War (and many suspect, are still being used for spying today).
Here's some more info:
(this BBC Magazine article has a few spooky clips of Numbers Stations broadcasts).
Samfan