Teslameter.

Teslameter lets you view current electromagnetic fields around you! To use, simply open Teslameter and bring your iPhone close to any ferromagnetic metal such as iron or steel, or any other magnetic object. Teslameter will measure the current magnetic fields on the X, Y, and Z axis in micro-teslas. In addition Teslameter displays a magnitude reading of (X^2+Y^2+Z^2).

How exactly does it work?
Since the iPhone 3GS/iPhone 4 includes a compass, Teslameter can measure the interference created by a ferromagnetic metal (or paranormal activity). Teslameter is NOT a true teslameter and shouldn't be used for anything but entertainment purposes.