If you had "pure" inertial mass, seems like you could make a "free" energy device that works off the Earth's rotation - you'd just need an (inertial) mass on a stick that turns an alternator around once per day. (edit: no, that would probably just settle at the highest possible orbit... hm)

Oh, also, airplanes that need almost no speed to start flying, but are still inertial-massive enough to not vibrate around on tiny bits of wind.