iMagneto is a functioning magnetometer. It uses the built-in hardware in your iPhone, iPad or iPod Touch to detect and display the strength of nearby magnetic fields.
The chip that powers iMagneto exists into your device primarily to power the compass. iMagneto puts it to a more novel use.
Magnetometers have many industrial and household uses. They are also used by ghost hunters and ufo enthusiasts to detect anomalous signals. Have fun!