Magnetised boots that let the wearer walk normally aboard a ship without artificial gravity, worn by the two killers who boarded Kronos One and later sought as evidence.
Gravity boots are a key clue in Star Trek VI: The Undiscovered Country. After torpedoes knock out the gravity aboard the Klingon flagship Kronos One, two figures in Starfleet pressure suits beam aboard and stride through the drifting crew to shoot Chancellor Gorkon. Only magnetic boots could have let them walk.
Believing the assassins came from the Enterprise-A, Spock orders the ship searched. Chekov finds a pair in the locker of Crewman Dax, but the suspicion collapses at once, since Dax's large webbed feet could never fit them.
The search eventually turns up the killers' uniforms, and then the bodies of the two men themselves, yeomen Burke and Samno, silenced by the conspirator who directed them, Lieutenant Valeris.