One of Alfred Hitchcock's earliest examples of a MacGuffin is the uranium sand that Claude Rains was smuggling in wine bottles in Notorious ("A vintage sand" is what Cary Grant called it). When studio execs told Hitchcock that movie audiences wouldn't understand why the uranium sand was so important, Hitchcock answered, "Then we'll make it uncut industrial diamonds. It doesn't matter what it is, as long as the villains want it. That's the MacGuffin, that's the motor that drives the plot."
The Green Destiny sword in Crouching Tiger, Hidden Dragon is a classical MacGuffin. While it does see a lot of combat and is a very good sword, its value is mostly ideological. It doesn't have any special abilities except of withstanding considerable abuse and being perfectly crafted.
Titanic (1997) is framed around the search for a diamond called Le Cur de la Mer (The Heart of the Sea/Ocean), which is quickly forgotten until the end of the story, when its owner throws it overboard so no one can have it.
The silver case in Ronin. The fact that the main characters' employers refuse to tell them what's inside the case is a minor plot point. We never find out either.
The stoner-flick Dude, Where's My Car? has two; first, the titular car, which serves primarily as a plot device to lead our half-baked heroes into zany misadventure after zany misadventure, and second, the Continuum Transfunctioner, a very mysterious and powerful device (Its mystery is only exceeded by its power.) being covertly fought over by two different alien races (which represent themselves as hot chicks and creepy Nordic dudes, respectively), a fight that the protagonists slowly find themselves caught in the middle of.
Mission: Impossible III features Ethan Hunt trying to keep a nasty weapons dealer from acquiring "The Rabbit's Foot", a small cylindrical kajigger that's assumed to contain some sort of biological weapon (though it's never explicitly stated as such). At the very end of the film, as Hunt leaves to enjoy his honeymoon, he asks his boss just what "The Rabbit's Foot" was, but his boss says he'll only tell him if he stays with the IMF. They all have a good laugh about it, and the movie ends. Shockingly some film reviewers (professional critics!) expressed outrage that they didn't get to find out what the all-important item was, suggesting unfamiliarity with the trope.
Mission: Impossible has a disc as the primary MacGuffin, though it was clearly defined as being a list of undercover IMF agents.
When the screenplay for Good Will Hunting was published as a book, director Gus Van Sant wrote a preface in which he admitted that Will's math talents were a MacGuffin: he doesn't solve a math problem the details of whose solution affect the plot (otherwise, the movie would be more a science-fiction story about the invention of fusion power, or whatever).
Mike's mother in My Own Private Idaho - the driving force for the plot is him trying to find his long-lost mother, but in the end he never does, even though he goes as far as Italy to find that she's just left. No MacGuffin, No Winner perhaps?
The titular proof in Proof. What it is doesn't matter, only whether Robert or his daughter Catherine was the one who proved it.
In Mel Brooks' High Anxiety, which contains parodies of numerous Hitchcock films, the lead character (who is terrified of heights) is checking into a hotel when the receptionist informs him that though the hotel had reserved him a lower-level floor, "a Mr. MacGuffin called and requested we change it to the 17th floor." Though MacGuffin is probably a reference to the villains stalking the main character, the name is never mentioned again.
Lampshaded in The Departed: "Our target: microprocessors. Yes, those. I don't know what they are, you don't know what they are, who gives a fuck?"
Done for humor in the Beatles' movie Help!. Ringo is given the ring of the goddess Kaili, which he can't get off and which various villains and bad guys are trying to get. One Mad Scientist comes out with the classic MacGuffin line: "With a ring like that, I could—dare I say it?—rule the world!!"
Wonder Woman (1974). A list of U.S. undercover agents stolen by the Big Bad and put up for sale to the highest bidder.
In the 1979 film The Double McGuffin (narrated by Orson Welles), a group of precocious children (including Lisa Whelchel) find a briefcase full of cash and run afoul of Ernie Borgnine and Lyle Alzado.
In Road to Rio, there are the mysterious Papers that have no bearing on the plot besides having an interesting Safe Cracking scene. Lampshaded when Bob Hope and Bing Crosby say that "the world must never know" their contents. At the end, when the papers have been recovered and they're about to be read, they get torn up instead, since they've served their dramatic purpose.
The jailbreak in Down By Law. We never find out how they got out, and it doesn't matter, because the movie is more concerned with the relationships between the characters (see also Noodle Implements).
James Cameron's Avatar. The Unobtanium actually has a legitimate purpose, as it is supposed to help make interstellar travel more practical. But since the movie isn't about the actual travel process, it honestly doesn't matter. It could just have been fruit that makes a better eyeglass polish.
What's Up, Tiger Lily? has several factions out to kill to possess the perfect egg salad recipe, stolen from a potentate who tells our hero "It is written that he who makes the best egg salad shall rule over heaven and earth. Don't ask me why egg salad, I have enough aggravation."
Monty Python and the Holy Grail. You could switch the Grail for an Ostrich Egg and the plot would not have been affected. You never even really SEE the Grail, just a Grail-shaped beacon! Bad Zoot.
The neutrinos in 2012 (it's mostly unintentional due to bad writing); not only do the planet's neutrinos "mutate and heat up the earth" and lead to "the end of the world", they never get another mention or fixed yet everything works out.
One World War II era spy film was already in the can when the atom bomb was dropped. It was yanked from release, to have its dialog somewhat reworked. A secret process, integral to the atom bomb, was replaced for — whatever MacGuffin the spies had looked for earlier.