~Virtual Reality Emulation.    
    
Can't afford a Virtual Reality Helmet and want to see (roughly)
what the difference is, then try the following :-

Obviously the game must have a VFX mode, so games this will work 
like games Magic Carpet, Descent and System Shock (but don't take my
word for it).

Load the game up in VFX mode.  There will be two side-by-side 
screens in gameplay.  (The flickering is due to page swapping.)  
These images represent the left and right images for your left and 
right eyes.  If you could somehow get each image to each eye, you 
will experience true 3D.

Get two small mirror surfaces.  Make sure they are small (not much 
wider than a butter knife), and mount them near your face (either 
over or under your eyes) so that the left mirror is reflecting the 
left image (and ONLY the left image) to your left eye, and similarly 
for the right eye.  Now, adjust them so that an object in the 
viewport will converge.  At this point, you should be seeing in true 
stereoscopy.

I wouldn't recommend playing like this for an extended period of 
time -- your eyes will get strained very easily.  Do it just to
experience the 3D effect.  If you can't live without it, buy a VR 
Helmet.