detect minute variations of a magnetic field, particularly the Earth magnetic field
http://www.geocities.com/CapeCanaveral/Lab/5185/circuit1.html#SENSITIVE
determining direction using the earth's weak magnetic field
http://www.edn.com/archives/1996/031496/06df2.htm
locates stray electromagnetic fields, response from 50Hz to 100kHz, will detect both audio and RF signals
http://www.mitedu.freeserve.co.uk/Circuits/Testgear/emmeter.htm
probes from very lof frequencies to 200 MHz
http://www.pupman.com/listarchives/1998/january/msg00601.html
detects very small currents such as those caused by leakage and changing electric fields
http://www.4qdtec.com/putpr.html#efld
It is excellent for tracing wiring behind walls, receiving audio from an induction transmitter, hearing lightning and other electric discharges, and monitoring a telephone or other device that produces an audio magnetic field
http://www.techlib.com/electronics/induction.html
a magneto-resistive sensor to detect small magnet displacements without resorting to a precision voltage reference
http://www.edn.com/archives/1994/080494/16di1.htm
uses radial type inductor as probe and responds well to low frequency changing magnetic and electric fields. headphones are used to for detection
http://www.mitedu.freeserve.co.uk/Circuits/Misc/emf.htm