EMF (electro-magnetic field) readers are very popular among paranormal investigators. I've seen them called "ghost detectors" before but I think that's a little over the edge. Sometimes paranormal phenomenon are observed in an area while the EMF remains normal, and other times the EMF goes crazy but nothing's visible. However, unusually high EMF readings almost always go hand in hand with ghost sightings, which suggests that ghosts are real since they can be detected by sensing equipment. As for the readers being easily fooled, that's why most good ghost hunters stay away from power lines, generators, and other sources of electromagnetic disturbances. Getting a strange EMF reading while standing in front of an electrical substation doesn't look too hot on a case report.
However, if you're standing in the middle of a field and that thing starts beeping like mad, get out the camera!
On an interesting side note, you don't need a pricey EMF reader to get results. I've heard of ghost hunters carrying compasses as a budget way of picking up on the EMF. Compass needles are really sensitive to electromagnetic fields (duh), although they can be thrown off just as easily.