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.
Step 3: Display Setup
The goal is to make the background as dark as possible. Play the movie you want to use full screen, then adjust as needed.
For the cabinet configuration, suspend the LCD monitor above the viewing area. Then create a Halloweenish looking scene inside, and then place the glass over it at an angle that will reflect the image to the viewer.
For the windows configuration, set the monitor on the floor pointing up, create your scary scene, then place a much larger piece of glass over it at an angle that will reflect the image towards the window.
In either setup, the bottom edge of the glass should be as close to the bottom edge of the monitor as possible. That will make the image float in front of your scene, instead of behind it.
Adjust the angle of the glass to control the height of the floating head.