I wanted to add a few like The Wicker Man or The Parallax View, but those are really more surprise endings you don't see coming and not quite the "trick" ending that turns around everything you saw beforehand.
Some others not mentioned yet:
3 Women (Robert Altman) Jacob's Ladder Open Your Eyes (Abres Los Ojos) (I suppose Vanilla Sky would fit this too) Session 9 Frailty Lucid Cello (if memory serves right...) Tale Of Two Sisters
Dear Frankie. Just when I thought I knew for sure exactly what type of movie it was going to be, it became something else entirely. Same for Million Dollar Baby.
Dystopian future of haves and have nots where food is scarce for the general population. Good Sci-Fi flick in the same vein as Omega Man, which also stars Heston.
So what would you call it? The film started as a typical boxing flick and seemed to be following the tried and true path that those types of films always seem to follow. All of a sudden it completely changed and took off in another direction and the ending became completely atypical for that type of film. I would call that a twist because the film took off in an unexpected direction.
What significance? That Joan Allen was so paranoid and bitter her husband laid dying out in the back yard for several days and that her children didn't blame her for it after they found out(especially since they were bitter people just like her)?