Some of the answers in this thread are... well, I won't say dumb, because I try my hardest not to argue matters of taste, but... some of the answers in this thread are... unexpected.
Se7en, for instance. Without the ending, that's nothing more than a police procedural with a somewhat Biblical spin. Still well written and well acted, in all likeliness, but not at all special. The thing that sets it apart is the absolute sucker punch of an ending in which, as a viewer, you're every bit as conflicted as Brad Pitt's character; you want him to do what he eventually does, but are sick with the realization he's allowing Spacey to complete his plan. Any other ending would have ing castrated the film. Including the ending they almost shot, frankly.
And No Country for Old Men has, easily, one of the most graceful endings in recent film. Bleak as , but with the same eerie stillness of the preceding two hours. Another case where I think the movie is made special by its final act.
American History X and There Will be Blood... I find those ones a little easier to let slide. I liked both endings, personally, and quite a lot, but can at least see the argument that perhaps AHX is a little melodramatic for its own good and that Daniel Day Lewis' final moments are amusingly hammy.