JingleJoe’s electromagnetic field detector looks quite awesome –
This device uses a circuit that I constructed myself to detect electromagnetic fields. All flowing electricity gives off an electromagnetic field, with this device you can hear them all! They can be musical and harsh, the variety of electromagnetic sounds you can detect is limited only by the number of electronic doohickeys you own! As an added bonus it also detects ghosts.
The device has three outputs for displaying the electromagnetic fields to your human senses: a speaker, a stereo quarter-inch jack socket and an analog meter.
More info over at the Hacked Gadgets forums.