One of the most oft-used and well known plot devices in Hollywood movies is the " MacGuffin ," the object that drives the story forward and is of vital important to both the heroes and villains even if the specifics of the object itself remain obscure or are unimportant. The treasure map is one of the earliest MacGuffins found in both literature and film; the crystal skulls in this week's new release Indiana Jones and the Kingdom of the Crystal Skull are just the latest examples of movie MacGuffins.

Dozens of great thrillers and adventure films -- including many of our favorites -- have revolved around MacGuffins, particularly the works of Alfred Hitchcock (who popularized the gimmick and its moniker). While MacGuffins are generally thought of as being meaningless and replaceable, for the purposes of this feature we're going to define them as the objects that motivate a film's characters and propels the narrative along.Here then are our Top 10 favorite movie MacGuffins. And be sure to let us know what your favorites are in the comments section below:All three Mission: Impossible movies have revolved around MacGuffins. In the first movie, both secret agent Ethan Hunt (Tom Cruise) and the villains were after a classified list containing the identities of double agents; the sequel saw Hunt out to obtain and destroy "Chimera," a genetically modified virus. But it was "The Rabbit's Foot" in Mission: Impossible III that best epitomizes the MacGuffin. Vile weapons dealer Owen Davian (Philip Seymour Hoffman) wants this mysterious, biohazardous item and it's up to Hunt to get it for him or else his fiancée Julia is dead meat. Hunt's mission takes him to China, where he must pull off an elaborate theft (we only see its set-up and aftermath).Khan killed for it. The Klingons wanted it. And Captain James T. Kirk lost his best friend, his son and his ship because of it. That's why the Genesis device, and the scientific secrets that allowed life to be created from lifelessness, has made its way onto our list. As created by Kirk's ex-flame Carol and his estranged son David Marcus, Genesis motivated the proceedings in both Star Trek II: The Wrath of Khan and Star Trek III: The Search for Spock , although one could argue that Spock's katra (soul) is just as important a MacGuffin in Trek III as Genesis.The Film Noir classic The Maltese Falcon follows private eye Sam Spade (Humphrey Bogart) as he becomes embroiled in an increasingly intricate plot revolving around a jewel-encrusted, black statuette that had been given in 1539 by the Knights of Malta to Spanish king Charles V. Spade would later refer to the eponymous statuette as "the stuff that dreams are made of." John Huston made his directing debut on this Dashiell Hammett adaptation, which influenced countless detective tales to follow.The plot of this classic was most recently "borrowed" by Mission: Impossible II. Cary Grant plays an intelligence agent who recruits Ingrid Bergman to infiltrate a spy ring in South America run by the urbane Claude Rains. It turns out that these villains are after plutonium in order to make an atom bomb, a plot that was unique at the time but has been done to death ever since. The real thrill of this film is the doomed love triangle that develops between Grant, Bergman, and Rains. The uranium is hidden somewhere on Rains' estate. OK, who wants wine?Although Kiss Me Deadly and Ronin both revolved around mysterious suitcases, Pulp Fiction offers perhaps the most tantalizing use of a case as the MacGuffin. What is inside the case that hitmen Vincent Vega (John Travolta) and Jules Winnfield (Samuel L. Jackson) have been sent to retrieve? We never know for sure, but the glow that emanates from it when Vincent opens to check inside suggests something either very valuable or very dangerous.