The Hexagon Key is an item in Fable and Fable: The Lost Chapters that is needed for the quest Hobbe Cave. The old woman at Rose Cottage will give it to the Hero after asking him to rescue her grandson from the Hobbe Caves. The key unlocks a door within the cave.
In Fable and Fable: The Lost Chapters, the door doesn't require the key to open before Hobbe Cave, making it entirely pointless.