HMC5883L 3-Axis Magnetometer
The HMC5883L  is a Triple-Axis Magnetometer or digital compass with an I2C output. The default resolution is about 0.92 milli-Gauss per unit, with a recommended range of ±1.3 Gauss.
It will operate from between 2.16V and 3.6V, so you need a regulator and logic-level conversion to operate it from a 5V board.
Although Adafruit and Sparkfun have discontinued their HMC5883L breakout boards, there are several Chinese boards available on eBay:
The I2C address is #x1E or 30.
Reading the magnetometer
The following routine s16 returns a signed 16-bit integer from two bytes, MSB first:
(defun s16 (s) (let ((d (logior (ash (read-byte s) 8) (read-byte s)))) (- d (ash (logand d #x8000) 1))))
Here's the routine xyz to read the HMC5883L:
(defun xyz () (with-i2c (s 30) (write-byte 2 s) ; Mode register (write-byte 1 s) ; Single measurement (restart-i2c s 6) (let ((x (s16 s)) (z (s16 s)) (y (s16 s))) (list x y z))))
It returns a list of three integers for the three axes, in the order (x y z). For example:
> (xyz) (-120 -186 -337)
- ^ HMC5883L Datasheet on Adafruit.