The Conet Project
http://www.irdial.com/conet.htm
Available: Online. Free. Legal. http://www.archive.org/details/ird059
One of the creepiest things I've heard. Voices speaking repeating patterns of gibbersih (sometimes backwards), voices dictating numbers, odd sounds and noises, tone signals, bits of music etc
Comes with quite an interesting pdf booklet on those Number Stations too.
Anyone caught similar transmissions? I remember playing with my FM radio at home some years ago and coming across a station that transmitted something like Morse code. Couldn't make anything out but the whole idea of an encrypted radio station was fascinating. Especially in Russia where the government forces all ogranisations to open codes and encryptions.