I was watching a bit of Casablanca last night, and was hit by the Macguffin, which is the "Letters of Transit" - an important item in the plot that doesn't otherwise make much sense. In Casablanca those letters, which never existed anywhere, are the device that allows Ilsa Lund and Victor Laslow to pass borders when he's wanted by the Nazis. The One Ring in LOTR, Private Ryan himself in Saving Private Ryan are other well known examples.
So, what are your favorite Macguffins?
So, what are your favorite Macguffins?