Charles Julius Guiteau shot James Garfield on July 2, 1881. Guiteau was a disturbed individual who started a fraudulant law company -- he flunked out of law school and failed the bar -- joined the Oneida Commune, and finally convinced himself that God wanted him to shoot Garfield.
Leon Frank Czolgosz shot William McKinley on September 6, 1901. Czolgosz was reportedly inspired by Gaetano Bresci's assassination of Italy's King Umberto I in 1900.
Lee Harvey Oswald shot John F. Kennedy on November 22, 1963. Oswald also tried to assassinate General Walker, a right-wing, failed gubernatorial candidate.
Lynnette "Squeaky" Fromme, a devotee of Charles Manson, tried to assassinate Gerald Ford on September 5, 1975. Not three weeks later, on September 22, Sara Jane Moore tried to shoot Ford. Both attempts were made in California -- first Sacremento, then San Francisco.
John Warnock Hinckley Jr. shot Ronald Reagan on March 30, 1981. Hinckley was motivated by an obsession for Jodie Foster whom he saw in repeated viewings of Taxi Driver.