Eye of Sauron

Following his loss of the One Ring, Sauron was unable to have a physical body to inhabit. The physical manifestation of Sauron in the Third Age was therefore as an eye, which could penetrate shadows and mists to find what it was seeking. While he could see much, he could not see all at once, a weakness Aragorn exploited by drawing Sauron's attention to himself by revealing his existance to Sauron through the Palantir of Orthanc, keeping Sauron from finding Frodo in his own land with his One Ring.