I'm sure Philip Zimbardo has done many interesting experiments, but the one that people mean when they refer to the 'Zimbardo Experiment' is the Stanford Prison Experiment. In 1971 Zimbardo (along with others) conducted an experiment to observe the effect of settings on people's behavior. Zimbardo had enlisted the help of 24 college students to act either the part of prison guards or prisoners. The students were assigned to these roles randomly.
On the morning the experiment started, the local Palo Alto police department assisted Zimbardo with the arrests of the 'prisoner' students. The students were (to their surprise) individually picked up by real police for violating Penal Codes 211, Armed Robbery, and Burglary. They were treated to full police procedure, including handcuffs. When the 'prisoners' arrived at the police station they were formally booked, finger printed, and taken to a holding cell where they were left blindfolded. They were next driven to the "Stanford County Jail", a mock up of a prison in the basement of the Stanford University psychology building. There they were searched, stripped naked, and deloused. They were then given a prison uniform (a smock) and chained at the ankle.
The students assigned as 'guards' received no training, only being told that this was a serious and possibly dangerous mission, and to deal with it as they saw fit. They were given khaki uniforms, billy clubs, and mirrored sunglasses.
The prisoners had to ask the guards before doing anything, from smoking a cigarettes to going to the bathroom. The guards were told to rouse the prisoners for 'counts' once per guard shift, waking them up from sleep if necessary. During these 'counts' the prisoners were asked for their identification numbers and quizzed on the prison rules. As the experiment went on, the guards spontaneously increased the length of the 'counts' from ten minutes up to over an hour, during which they mocked and ridiculed the prisoners.
"I was pretty surprised at myself... I made them call each other names and clean out the toilets with their bare hands. I practically considered the prisoners cattle, and I kept thinking: 'I have to watch out for them in case they try something'"
-- Guard M
By the second day, the 'prisoners' rebelled, ripping off their identification numbers and barricading themselves inside the cells. This lasted until the morning shift came in, replacing the night shift. The 'guards' acted in force, calling in reinforcements (other 'guards' who were off shift). The guards first used freezing fire extinguisher spray to force the prisoners away from the cell doors. They then broke into the cells, stripped the prisoners naked, and put the ringleaders of the rebellion into solitary confinement.
The 'guards' then set up a 'privileged cell', in which the good prisoners could stay. The privileges included being allowed to keep their beds, wear their uniforms, wash, and eat regularly. The guards next switched the prisoners in the bad cell with the prisoners in the good cell. I'm not sure what they expected to gain with this tactic, but it confused the prisoners. This is apparently not uncommon in real prisons, and is intended to turn the prisoners against each other. Soon the guards were using bathroom privileges (as opposed to a bucket in the cell) and smoking privileges to help control the prisoners.
36 hours after the experiment started, the first prisoner broke down, suffering emotional disturbance, disorganized thinking, uncontrollable crying, and rage. After Zimbardo and the other experimenters convinced themselves that he wasn't faking (why should that matter? The students were supposed to be able to leave the experiment whenever they wanted. The experimenters were already getting too wrapped up in the experiment themselves), they let him go.
The next day was the visiting day. The prisoners' families and friends could come and see how the prisoners were doing. The prisoners were shaved and washed, fed a good dinner, and made to clean their cells. Despite some worried parents, all the remaining prisoners stayed.
Next were rumors of an escape plot. The released prisoner was rumored to be coming with helpers to release the prisoners. Instead of studying it, the experimenters took up arms to stop it. It is also at this point that Zimbardo frankly admits that he and the other experimenters were too wrapped up in the experiment. The prisoners were chained, blindfolded, and marched up to a new prison on the fifth floor. When the rescuers arrived, only Zimbardo would be in the basement, and would tell them that the experiment had been canceled. The imagined rescuers never showed up.
The guards were angry. Endless push-ups, jumping jacks, and meaningless repetitive tasks (such as cleaning the toilets) were assigned to punish the prisoners. The situation did not improve, and on the sixth day after the experiment began Zimbardo called an end to the whole thing - 8 days earlier than planned.
This is an incomplete account. For the whole story, check out here and here.
The Stanford Prison experiment has been used to illustrate a number of different ideas. Zimbardo concluded that environment and social expectations are a controlling factor in how we behave, and there certainly are other experiments that support this viewpoint. However, you are just as likely, if not more likely, to see this as an example of why it is important to consider the ethical implications of studies; by Zimbardo's own account, he did not stop the study until confronted by a graduate student who saw that things were clearly out of hand (as a side note, he was dating the student, and would later marry her). It is also frequently noted that the experiment was poorly designed, and was essentially an interesting way to gather anecdotal data; after all, it had no control group, no formal way to quantify and analyze data, and most certainly was not double blind. Even so, it remains an interesting example of what happens when humans are allowed to be humans... even when those humans are trained scientists.
For participating in this experiment the students were paid $15.00 a day.