Eye of Mordor

From his high place in Barad-dûr, Sauron looked out across the lands of Middle-earth through the Window of the Eye. His power of vision was very great, and he could watch events unfolding hundreds of miles from his Dark Tower. He was not all-seeing, though: he could only study one thing at a time, and the sight of his Eye could be distracted, or blinded by shadow. It was through these failings that the Wise were able to send Sauron's Ring into the heart of his own Dark Land.

The powers of the Eye of Mordor seem very similar to those of the palantíri, the Seeing-stones, and it is perhaps no coincidence that Sauron held one of these under his control. There are other indications of a connection between the two. For example, when Aragorn used a palantír to reveal himself to Sauron, '...the Eye turned inward, pondering tidings of doubt and danger: a bright sword, and a stern and kingly face it saw...' (The Return of the King VI 2, The Land of Shadow). So, it is clear that the power of the Eye was somehow connected to the Ithil-stone in Sauron's power, but whether the Stone granted him the power of the Eye, or merely enhanced his own natural powers, is open to question.