ELIZA
The first chatbot, written by Joseph Weizenbaum at MIT in 1964–1966. The famous DOCTOR script imitates a Rogerian therapist by reflecting whatever you just wrote back at you as a question.
The technique is pure keyword pattern matching with simple rewrite rules — no understanding, no memory, a few hundred lines. "I hate my brother" becomes "Tell me more about your family". Weizenbaum was deeply disturbed that people, including his own secretary, wanted to be left alone with the program and confided real things in it despite knowing exactly how it worked. That phenomenon has been called the ELIZA effect ever since: our tendency to attribute understanding to systems that merely produce plausible text. His book Computer Power and Human Reason (1976) remains uncomfortably current.