TheNewHobbyist has posted an Instructable for re-creating the "Singing Busts" effect from the Disney Haunted Mansion rides, using an Arduino to control the effect so that it springs to life when trick-or-treaters step up to your porch.
After I started learning to program my Ardunio this evolved into a photocell actuated video "on demand" Halloween display. I used an example I found on Arkadian.eu to control a Dell Mini 9 netbook running AutoHotKey. When the photocell switch is tripped the Arduino sends a serial message to the Dell laptop which is converted to a keystroke by AAC Keys which in turn triggers the below AutoHotKey Script and finally plays back using VLC. The "AAC Keys" could probably be replaced by setting up your Arduino as a USB HID but at the time this was all a foreign language to me.