I was at first just thinking of Novella length works but let me mention this example even though it's much longer. Well actually I suppose the "Enemy Mine" portion is a novella but it is part of a larger collection called the "Enemy Papers" by Barry Longyear.
I saw the movie when it first came out and enjoyed it, although some of it seemed hollywood predictable and the f/x were so/so, I felt there were enough moments and depth of characterization to make the film redeemable. The relationship between Davidge and Jerry came through well, the two combatants coming together to form a bond is a common and old enough theme but I thought the film, the actors did a good andn interesting job of it.
But I was only dimly aware the film was based on a novel.
Many, many years later I read the entire "Enemy Papers" collection and the depth and complexity of the overall story is a powerful and exciting story.
It's been a while since I've read the stories so I can't list details of the deviations between the Film and the Novella but the ending is quiet different none of the hollywood heroics and much less daring-do if you know what I mean. But the spirit is there unfortunately it not as deep as the novella because the film makers clearly had no intention of following the story through which covers a much larger scope - peace between peoples and a more complex and full set of characters and interesting relationships annd a much deeper relationship between Davidge and Jerry's decendantns.
http://en.wikipedia.org/wiki/The_Enemy_PapersOn Edit: Just found this in my bookmarks:
http://www.sff.net/people/bblongyear/MoviesTheTruthRunDrac.html