The grand-daddy of conspiracy theories. US President John (Jack) F. Kennedy was shot in Dealey Plaza, Dallas on 22nd Nov 1963 as his open-top car was traversing Elm St. The official Warren Commission report into his death pointed to a lone-gunman, Lee Harvey Oswald, as being the sole culprit, having fired three shots from a window on the nearby Texas Book Depository building. Several other investigations done by Dallas police, the FBI and other Government bodies throughout the years have similarly concluded this fact, however some reports, done years later, did conclude that there was a "probable conspiracy" but even then, there were dissenting voices in the authors of these reports and the conclusions were usually based on anomalies seemingly heard on audio recordings of the event. There has never been any good evidence to show that anyone other than Oswald shot Kennedy.
Despite this the murder has been the subject of numerous books, movies and articles all with different theories as to what happened, but all essentially refuting the "official version of events". Oswald’s links to Russia and Cuba, the fact that the event was caught vividly on Abraham Zapruder's colour film, numerous other cine-film footage and still photographs along with the fact that Oswald was shot very soon after - again caught on film and photograph - by small-time crook Jack Ruby with supposed Mafia connections, provided lots of avenues for speculation and supposition.
The murder has led to several terms closely associated with conspiracies: terms such as Grassy Knoll, Umbarella Man, Second Shooter and Magic Bullet have all entered common parlance. Most of the conspiracies simply throw doubt on the official narrative and then speculate endlessly from there.