An interesting portion of the radio spectrum lies below the normal AM broadcast band. Amateurs are allowed to operate weak transmitters in the region of 160-190khz.
Such amateurs call themselves lowfers, and experimenters are doing interesting work in
propagation, antenna design, modulation and demodulation techniques, and signal detection. The Long Wave Club of America is an interesting place to start.

The NIST in Boulder Colorado mainains a low frequency radio station designated WWVB. It broadcasts
a very accurate time signal on 60khz. I had a radio from Oregon Scientific that could receive these signals and set itself to the correct time. I’ve often thought it would be interesting to make a
highly accurate clock that would synch with this source, although recently synching to
GPS satelliteshas become more popular.

Below 10khz or so, natural radio sources dominate, mostly in the form of sferics: low frequency radio signals that correspond to lightning. NASA runs the very nice
INSPIRE site. From their website:

INSPIRE is a non-profit scientific, educational corporation whose objective is to bring the excitement of observing natural and manmade radio waves in the audio region to high school students. Underlying this objective is the conviction that science and technology are the underpinnings of our modern society, and that only with an understanding of science and technology can people make correct decisions in their lives, public, professional, and private. Stimulating students to learn and understand science and technology is key to them fulfilling their potential in the best interests of our society.

Very cool. They rebroadcast the output of a VLF receiver via icecast (which Unix weanies can
play using mpg123).
Just this morning I heard a couple of tweeks, in between a lot of bacon frying.