ELIZA
A famous program by Joseph Weizenbaum, which simulated a Rogerian psychoanalyst by rephrasing many of the patient's statements as questions and posing them to the patient. It worked by simple pattern recognition and substitution of key words into canned phrases. It was so convincing, however, that there are many anecdotes about people becoming very emotionally caught up in dealing with ELIZA. All this was due to people's tendency to attach to words meanings which the computer never put there.
See also ELIZA effect.