[Sam The Egg]

Since they weren't in horror much (Mostly Noir), this won't apply exclusively to horror movies. What has been your favorite MacGuffin in movies? In case you're not familiar with what it is, I'll explain it. The MacGuffin (a phrase coined by Alfred Hitchcock) is an element in a movie that is placed to drive the logic of the plot, but which actually serves no further purpose.

An example (and my pick) is the Letters of transit in Casablanca. They didn't really serve any purpose except to move the plot along.