These are numbers stations. A spy relic from the cold war. Many countries still have them in place, including our own. You can usually find these signals between the spectrums. The common belief is that the send out information that's associated with a one-time-use pad. Once they use the numbers to decode the message, they can't use that pad any longer. There's a large collection of these listed under The Conet Project (The End Project, in Czech), which is a downloadable album that can be purchased from Amazon.com
http://en.wikipedia.org/wiki/Numbers_stations
http://www.archive.org/details/ird059