Extremely Low Frequency Electromagnetic Field Sensing
An Arduino board can do many things, and Didier has described in detail methods for measuring extremely low frequency electromagnetic fields using just an Arduino board, some wire and a few special libraries.
You could also display the results using a grapic LCD unit to make a portable version of this simple test equipment. For more information, descriptions and the Arduino sketch, visit Didier's blog. To get started with your own measurements, consider ourFreetronics LeoStick - one of the smallest Arduino-compatibles on the market which also includes an onboard RGB LED and piezo buzzer: