Zeus plugs were tools used in the maintenance of TARDISes.
The Fourth Doctor decided he didn't need the plugs to repair the predicted damage to his TARDIS' thermocouplings. (TV: The Hand of Fear) He later used a Zeus plug to incapacitate a Movellan. (AUDIO: The Movellan Grave)
The Tenth Doctor needed his Zeus plugs to close the time windows aboard the SS Madame de Pompdour. He was unable to find them but recalled using them moments before as castanets at a French party. (TV: The Girl in the Fireplace)