Leslie points me to the Grand Illusions Shop, which has some pretty awesome stuff. I especially like the non-transitive dice game:
Ask your opponent to select any one of the four dice. You select another and both dice are thrown, at the same time, a predetermined number of times, to see who gets the highest number on each throw, and hence wins that throw. In a game of ‘The Best of Ten Throws’ you will almost always have more wins. Invite the player to choose another die – perhaps your ‘winner’ – leaving you to select another for yourself and play again. Again you will win. Whichever die your opponent selects, your choice, in a longer run of ten or more throws, will always win.