The prisoner's dilemma is a classic game theory problem. Two prisoners are being separately interviewed by the police. If one confeses and the other does not they one who confesses gets a reduced sentance of 1 year and the other gets a very long sentence of 10 years, if both confess they both get 5 years, but if neither confess they are freed. The most risk-free choice for each prisoner is to confess (worst case 5 years), but the best thing is for neither to confess. This is an example of a non-zero-sum game as the rewards/penalties are not a strict trade-off between the parties. It also shows the importance of communication.
Used on pages 226, 228, 378, 564