I believe some cheap devices depend on magnetometers to sense their attitude. They're paired with a gps so they know what the magnetic field is supposed to look like wherever "here" is but I wouldn't count on it at extreme latitudes and FL350.
For a cheap demonstrator/toy, there's a free bubble-level app for Android phones that have the appropriate hardware. I'd say it's accurate to a minor fraction of a degree. Not so good for CAT III, but (especially if the sensor is magnetic) great for noticing that your deck angle is 15 degrees when the million-dollar computers have just had a domestic spat and thrown up in your lap. Not that anyone would whip out their cellphone at that moment and start fiddling with it.
At a more professional level, this thing is about USD5000:
Inertial Navigation System: INS GPS, MIDG INS/GPS, IMU Global Positioning System, unmanned vehicles, servo controller. See also rc_uav_autopilot.html on the same site; don't know the price on that, but more than bare MIDG.