Clockwork Orange. The movie stops before the final chapter of the book.
It may be a stretch, but I, Robot didn’t quite end the same as the book. 
The ending of The African Queen is MUCH more downbeat than the movie. I’m sorry I ever read the book, and have mercifully forgotten the details.
Minority Report
Quite a bit is different because it is a short story so stuff has to be added.
However the main difference is that the Tom Cruise character, in the story, commits the murder as predicted. He does this because he feels the group is more important than the individual. The movie takes the opposite view and changes the ending.
Another great adaptation is Field of Dreams, which was adapted from Shoeless Joe Jackson Comes to Iowa. Not only is the title better but they dropped Ray’s twin brother, and it has a much more satifying ending. (There is no ‘Dad do you want to have a catch’ bit in the book nor is there a huge line of cars coming in to save the farm.)
There are so many of them, it’s hard to know where to start. I have to agree with most posted above. I also note that while this my be true:
You shouldn’t single out Disney on this. I don’ know of any version of The Hunchback of Notre Dame that’s faithful to the book (and there have been a lot of them.) And I don’t blame them for changing the ending of The ittle Mermaid, or of The Steadfast Tin Soldier (in Fantasia 2000) – that Hans christian Anderson went out of his way to put downbeat, tear-jerker endings on stories that weren’t traditional stories, but his own creations. In fact, especially with the Tin Soldier, the sad ending seems like it’s more “tacked on” than a happy ending would be.
As we’ve noted befoe, virtually no one has been completely faithful in adapting Dracula to the screen, but that novel is so overblown , with its huge and irrelevant cast that it’ not surprising that every adaptation shaves down the list and condenses characters. Until recently, no one has ben happy showing Frankenstein as the downbeat story it is (more downbeat than Hans Christian Anderson), but even the more recent versions have verred from being completely faithful.
No adaptation of Heinlein has been faithful. Starship Troopers should die thousand deaths, but The Puppet Masters made huge changes (not setting it in the future, for sarters), and Destination Moon didn’t resemble Rocketship Galileo (although that’s probably for the best, and Heinlein himself wrote the screenplay).
Similarly, no adaptation of Philip K. Dick’s has been faithful, reither. It makes me seriously wonder why they even bother buying the property, because the canges have been so profound. Minority Report has been mentioned, but how abot Bladerunner, or Total Recall (which, I’ve argued many times, exhausts the original short story possibiities in the first 20 minutes, then steals from Robert Sheckley’s “The Status Civilization” for the rest), or Screamers (Ostensibly based on “Second Variety”) or Paycheck?
Try reading This Island Earth by Raymnd F. Jones sometime. The movie has irtually nohing to d with the novel/ Even where they seem to be close, they totally miss the point. In building the nterociter, scientist Cal Meacham (lov that name!) has to show he can read between the lines, figure out how things go together, even rebuild parts. In the movie, it’s as if he gets an alien Heathkit, plans and all, and just has to solder it together.
John Irving’s A Prayer for Owen Meany / movie Simon Birch.
20,000 Leagues Under the Sea.
Book: Nemo survives, everybody returns to status quo ante.
Movie: Nemo dies, Nautilus sinks, Arronax ponders the Human Condition.
Suspicion (1941), the Alfred Hitchcock movie based on the brilliant psychological suspense novel Before the Fact by Francis Iles. Censorship totally changed the ending, making it into a stupid, neutered film that made no sense.
Also, Double Indemnity (1944)—a corking good film, but the original by James M. Cain involved Satan worship and a completely different—and creepier—ending.
About a Boy. There’s no talent show in the book. Instead, Marcus and Ellie take a train to Cambridge, but get out early. Ellie smashes a window and they get arrested. Also, Will and Rachel don’t break up and get back together in the book. Rachel’s surprised that Will isn’t Marcus’s father, but she isn’t bothered enough to break up with them.
Actually, I have a paper on this subject due Friday.
Man On Fire
[spoiler]In the book the Pinta dies, Creasy lives and after killing everyone who was connected with Pinta’s kidnapping takes off to Greece I think to live with a woman he fell in love with during his recovery.
In the movie, Creasy discovers Pinta is still alive and trades himself for her. Pinta returns to her mother, and Creasy is killed by the bad guys (the ringleader is then killed by the cop as payback for Creasy’s death)[/spoiler]
The rest was pretty faithful to the book as I recall, except that the action takes place in Italy not Mexico.
In the short story that All About Eve is based on, Eve runs off at the end with the husband of the woman who first helped her (I forget if he’s the playwright or the director). In the movie, Eve tries to lure away both the director and playwright from their respective wife/fiancee, but fails, and has a sort of comeuppance via George Sanders.
Granted, there’s a really good reason for that. For the longest time, the American version of the book didn’t have the final chapter and that’s the version kubrick read. I don’t think he realized until after he made the movie that the 21st chapter existed.
No-one’s mentioned The Naked Lunch yet? I would say that the movie and book are pretty different in both beginning, middle and end. (ie the film had these three elements, the book didn’t)
Wow. Every one I came in here to mention has already been mentioned. The two worst offenders were Time Machine and Minority Report (IMHO). Time Machine became a sappy-shit love story* and Minority Report had such a different feel that when I read the short story six months after the movie I almost didn’t recognize it.
*I’m a girl. Why, oh why…I think I’m actually going to make a new thread about this complaint.
My point that I was going to make was why does every director have to add stupid sappy love scenes…
I thought that in the book, the Nautilus was last seen going into maelstorm when the professor and co. abandoned ship and at the end they weren’t sure if Nemo survived or not.
I’m not bringing Mysterious Island into this because the timeline of Mysterious Island makes no sense at all in respect to 20,000 leagues.
Memoirs of an Invisible Man didn’t use the ending in the book. Nor much of the beginning or middle.
Another John Irving book “The World According to Garp”–Garp dies in the book, but the movie was obviously filmed with a sequel in mind.
Cyrano de Bergerac -> Roxanne Nice updating of Rostand’s play, 16th Century French soldier becomes 20th Century American fireman. Most of the plot is similar, even many of the jokes are faithful to the original. But the ending is totally different. The play is a tragedy (albeit with more than its share of comic relief), the film is a comedy.
Not only that, but in the book Forrest is an abrasive, foul-mouthed, hornball (very unlike the innocent character from the movie) who ends up becoming an astronaut with an ape in his crew. He crash lands in a jungle inhabited by cannibals. At some point, I think he meets up with Racquel Welch. VERY different in tone and message from the movie.
Cujo, IIRC. I don’t think the kid died in the movie.
Cal , Destination Moon was not based on the book Rocket Ship Galileo. It was based on a short story of Heinlein’s titled, umm, “Destination Moon.” Not sure what all anthologies it’s in, but I read it in Requiem , a book of lesser known Heinlein works, plus essays about RAH, collected after he died. Along with the story, Heinlein went into great detail explaining how the movie was made, quite interesting. The movie is as faithful to the plot (and science known at the time) as it could be.