Eye of Sauron
From the highest tower of Barad-dûr, Sauron kept an unceasing watch on the lands and kingdoms of Middle-earth. The 'Eye of Sauron' is a reference to this unsleeping vigilance. In the book it was described as a sort of red light from an upper window. In Peter Jackson's Lord of the Rings film version it was portrayed more literally as an eye, and had it moving between the pinnacles of the tower.