Hide your Monitor like a Spy. - This 8-Bit Life
Here’s a deceptively simple hack that anyone with the guts to crack open their monitor can do. Instructables user [dimovi] created his own privacy monitor by simply removing the polarizing film from his monitor so that it is unable to display anything other than a flat, white image. He then removed the lenses from a pair of cheap 3D glasses and replaced them with cut-outs made from the polarizing film. This allows anyone wearing the glasses to see the images displayed on the monitor. Anyone not wearing them will be unable to see the image. Simple and absolutely brilliant!
Check the video below to see it in action.