You've heard about the Stanford Prison Experiment. Now you can watch the video on YouTube. The experiment was conducted in the basement of the Stanford Department of Psychology in 1971. It gives some insight into how atrocities like the Abu Ghraib prisoner abuse scandal can happen. Interestingly enough, according to Wikipedia, "the study was funded by the US Navy to explain conflict in its prison system and the Marine Corps." Video: [1], [2], [3], [4], [5]. If you don't want to watch all of the videos, the last video in the sequence (#5) is a round-up of lessons from the experiment.
Update: Jake Young at Pure Pedantry was a teaching assistant for Dr. Phillip Zimbardo at Stanford not too long ago. Young says that it was Zimbardo's graduate student Christina Maslach who convinced him to stop the experiment by the sixth day. Zimbardo was dating his graduate student, Ms. Maslach at the time, and now they are married.