Magic Eye is a series of books that feature "autostereograms" which allow people to see 3D images by focusing on 2D patterns. The technical name for these images is "SIRDS" or "single image random dot stereo" images. One enterprising gamer has applied the algorithm for SIRDS to Quake II, creating a Quake II mod called AbSIRD. Once you get your eyes adjusted to the pattern you'll be able to play the entirety of Quake II in a kind of faux 3D, without the aid of 3D glasses.