Got an old LCD monitor sitting around? With some hands-on hacking, you can make a screen that's only viewable with the right pair of glasses.You can remove the polarised and anti-glare films from the inside of the monitor's glass surface and reassemble it; this will make the screen look bright white to the naked eye. To see the actual display you just use paint thinner to remove the anti-glare element from the polarisation film and cut that to match the frames of an old pair of glasses. This produces a monitor that can only be seen when you're wearing the polarisation filter glasses.
Instructables user dimovi provides step-by-step instructions and photos that describe how to remove the polarisation filter from the display and cut that filter to make lenses for disposable 3D glasses or old sunglasses. I'm curious to know if regular polarised sunglasses would also be able to view the hacked display.