Here's a link to the full Instructable on how to make the Mr. Hyde Halloween robotic top hat.
This reminds me: I finally got around to replacing the rubber finger puppet on my trick top hat with a little wooden monster of my own design. He has articulated arms that move as a result of the monster's motion. The hand on the hat brim works particularly well, appearing to grab the rim of the hat as the monster emerges. Here's what the trick top hat looks like with the new monster:
Plans for my low-tech (but still incredibly fun) trick top hat can be found in MAKE Magazine's Special Halloween Edition.