I've actually used this one several times. Got interesting results.

NPC Of Three Wishes

An old man who does not speak. He is only capable of answering yes or no questions by nodding/shaking his head. He knows everything there is to know, but it is up to the questioner (the PCs) to ask the questions. This NPC is willing to travel with the party if asked. Only the party, deities, and particular unique beings know he is there. Whenever any player, in or out of character, says anything along with the word "wish" that wish is immediately granted with least harm to the wisher as possible. After the third wish is granted, the old man disappears.

My favorite wish: In a 2E game the party met a werewolf who wasn't attacking. While unknowing the nature of the old man, the paladin player sighed "I wish I could cure you." Wish granted. Paladin could expend a Remove Disease and cure the werewolf of his lycanthropy.