Pavlov's 1902 experiment with salivation in dogs:
Skinner's 1938 experiments on rats:
Influential but unethical experiments
Milgram's 1963 Obedience to Authority experiment - journal article on the ethics of deception in the Milgram experiment
Latané and Darley's 1968 Bystander Effect experiment - a journal article that examines the ethical issues of 3 specific psychology experiments, including Latané and Darley's.
Zimbardo's 1971 Stanford Prison experiment - Stanford University Alumi magazine article revisiting the experiment 40 years on, including ethical considerations and interviews with some original participants.
Asch's 1951 Conformity experiment - Psychology website that details the experiment and highlights the ethical problems with it.