HMC5883L Triple-axis Magnetometer Board
The HMC5883L is a Magnetometer Sensor capable of finding the earth's magnetic poles (North, South, West and East) by detecting the magnetic field of the earth.
The sensor measures the earths magnetic field in X, Y and Z direction and outputs the results through I2C communication which can then be read by any Microprocessor/Microcontroller. You can also make use of the libraries if you are planning to interface this sensor with Arduino.
Note that, the module uses QMC5883L IC instead of the HMC5883L. Make sure you refer to the right datasheet/library.