This classic animatronic prop is "de rigueur" for any serious haunter.
This ghost prop was pioneered by Doug Ferguson of Phantasmechanics, who made it public domain in 1997. It uses a slow motor and a simple system of pulleys to create an eerie, ghostly motion that literally stops cars in front of homes. Open source and cheap to make, it’s become a haunting classic.
CAUTION! This project uses enough electricity to kill you. Please be careful. Electricity can be fun and safe if kept inside its insulating containers.
More on the FCG: phantasmechanics.com