cables and pulleys!
Variable resistance implies that the load against the prime movers varies through the range of motion of the movement. A 10 pound dumbell is a constant resistance through the ROM. Machines that employ a cam are considered to modify the effective load against the prime movers through the ROM. If you are trained and skilled, perhaps the most effective and true variable resistance is manual resistance against prime movers through the ROM.
Cams, Cables, Pulleys, Resistance Tubing,