Anor Stone

The Anor Stone was a Palantir secretly owned by Denethor, Steward of Gondor. While it is never revealed in the films (except for tie-in products), its presence is hinted as Denethor seems informed on secret affairs happening miles away from his dominion, such as the finding of the One Ring, the coming of Aragorn and so on. Speaking to Gandalf, Denethor refers to it obliquely as "The Eyes of the White Tower."
Saruman seems to allude to it as he speaks of something that "festers in the heart of Middle Earth." Indeed, it is through the visions supplied by this stone that Sauron fed Denethor's despair, which conspired to drive him to madness.