Sigmund Freud (1856-1939) is known around the world as the "Father of Psychoanalysis," and for good reason. If anything, Freud's first patient was himself. A sufferer of psychosomatic symptoms, Freud diagnosed himself as having a repressed antagonism against his father. From there, Freud began to build on his now famous concepts of the unconscious, infantile sexuality and repression. And of course, there's his famous theory on the structure of the mind, which has made Id and Ego a commonly used part of the English lexicon.
In addition to all but creating a new field of science, Freud also contributed to entire industries. One of the first to try to analyze dreams, Freud's work has led patients in search of psychological explanations for various physical and mental symptoms and phenomena.