Eyeballs are organs. Like other separated body parts, they are sometimes used as Halloween decorations or as part of scary places, like haunted houses. In a haunted house or mad scientist's lab, you might find some "eyes" staring at you from a jar.
A popular Halloween game, which is featured in the episode of The Simpsons "Treehouse of Horror III", involves participants being told to touch objects which they cannot see, either because the room is darkened, the participants are blindfolded or the objects are covered up. The participants are told that the objects are body parts, often those of a dead witch or other frightening character. In reality, the objects are simply foodstuffs and other ordinary household items. Peeled grapes are often substituted for eyeballs in this game.
- Wikipedia article on eyes