"World War II Weapon: Monopoly With Real Money"

In 1941, the British Secret Service asked the game’s British licensee John Waddington Ltd. to add secret extras to some sets, which had become standard elements of the aid packages that the Red Cross delivered to allied prisoners of war. Along with the usual dog, top hat and and thimble, the sets had a metal file, compass, and silk maps of safe houses (silk, because it folds into small spaces and unfolds silently). Even better, real French, German and Italian currency was hidden underneath the game’s fake money. Departing allied soldiers and pilots were told that if they were captured they should look out for the special editions, identified by a red dot in the Free Parking space. Any sets remaining in the U.K. were destroyed after the war. Of the 35,000 prisoners of war who escaped German prison camps by the end of the war, “more than a few of those certainly owe their breakout to the classic board game,” says [historian Brian] McMahon.

More details here. (Via Bruce Schneier.)