The Hexagon Key is an item in Fable, Fable: The Lost Chapters and Fable Anniversary 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, Fable: The Lost Chapters and Fable Anniversary, the door doesn't require the key to open before Hobbe Cave, making it entirely pointless.