Head mounted displays with head tracking.
During the 90's Virtual Reality was a huge buzz word, and there were a few companies taking it from the battlefield to the living room, but for a grand, it was just too much to stomach, and computers of the time couldn't render two 3d images at the same time with a quality high enough that was truly immersive.
Although technically, the Occulus Rift may change that very soon, and Sony has a HMD on the market now, and Nvidia is considering it. So, it may finally be coming.