Rotor RS4X crank system. IMO the greatest uphill-off-road-constant-traction-enhancing-component after knobby tires and adjustable seatposts. Yet they stopped making them a couple years ago. Also highly useful for partial amputees, who are many in number after the last couple wars. So if the equivalent product known as Powerpace doesn't flourish, and Rotor doesn't reinvent the system, and no-one else comes along with an equivalent, I expect they'll be rare and quite sought-after by a small number of people in a decade or two. Unlike Q-rings which are legion, and don't have quite the same utility.