This instructable uses parts from a hardware store and an Arduino to make a motion activated spider. Looks like a fun project to scare the neighborhood kiddies.

I wanted to create a motion-activated spider to put on my door for Halloween. It drops down when visitors approach the entryway, activates a spot light to draw their attention to the spider, gives them a scare, then automatically retracts and resets itself, waiting to prey upon its next victim.

More about DIY: Motion Activated Halloween Spider