It's a great addition to your Halloween decorations!
Watch the video for the overview. (The camera can't do this illusion the justice it deserves.)
Step 1: Supplies
1. A piece of glass.
The size of the glass is relative to the size of the area where the illusion will be viewed.
2. A monitor (and computer, of course).
Either an LCD or CRT monitor will work. CRT monitors tend to give a better reflection, but an LCD monitor can be used in a much smaller spot.
The LCD monitor I used had a removable stand. That made it much easier to hide in the cabinet.
3. Floating head movie.
I recommend Big Scream TV.
Step 2: Location, Location, Location
For this instructable, I used a bookshelf/cabinet and the front window of my house for the trick-or-treaters to enjoy.
Projectors are relatively cheap these days. You could easily project the image onto a piece of glass 30 or 40 feet away. The projector would need to be located very close to the path of the intended viewer, however.
A great example of using a projector would be on an eave of your house. The glass could be suspended at an angle towards the front of the house, and then the image would be projected on it from a hidden spot on the roof. As the trick-or-treaters walked up to the house, they would be greeted with a creepy floating head on your roof.