I picked up a Model RBL-2 National (Navy Bureau of Ships, Order April 30, 1942) VLF receiver at the Newark (Drumlins), NY hamfest on 4-9-05. It works great and I have been having fun exploring the spectrum below 500 kHz.
There's plenty of info on the Intranet about the Extremely Low Freq (ELF 0-3 kHz), Very Low Freq (VLF 3-30 kHZ), Low Freq (LF 30-300 kHz), and Medium Freq (300 kHz - 3 MHz) bands. You may hear Navy transmissions at 76 Hz, Natural Radio (Whistlers, Tweeks, Dawn Chorus, etc) 0 to 40 kHz, WWVB Time Station at 60 kHz, and Loran C at 100 kHz. The License-Free "Lowfer" band is 160 - 190 kHz and the Foreign AM Broadcast is 150 - 280 kHz. Navigation Beacons dot the globe and can be heard generally in the 190 - 400 kHz area. When you copy the CW Id, check out these websites to see where they are:
http://worldaerodata.com/http://airnav.com/navaids/During this week, I've been hearing a Broadcast Station from Allouis, France. They transmit a 2000 KW signal! Try listening around 9 to 10:30 PM. It's a lot of fun and a whole new world in the "basement" of the spectrum.