Not necessarily useless, but funny...
Schrödinger's Box: When a feline mammal is placed inside, Schrödinger's box may or may not poison it.
Heisenberg's Uncertain Arrows: Anyone observing Heisenberg's Uncertain Arrows can either discern their location or their velocity (50/50 chance), but not both. Anyone struck by Heisenburg's Uncertain Arrows is, thus, flat-footed against the attack.
Rods of Pythagoras: These two rods are connected at a 90 degree angle, and each telescopes between 3 inches and 4 feet. When the command word is spoken, a line of yellow light runs between the unconnected ends of the rod; the length of this line is always equal to the square root of the sum of the squares of the lengths of the Rods of Pythagoras.