Movies about people going nuts

Dave Schuler poses an interesting question over at The Glittering Eye:

What are the best movies, among genre films or straight drama, in which the main character starts sane and loses his or her mind during the course of the film?

Being both a lover of movies and a crazy person, I feel compelled to weigh in on this topic.  Several good examples were already mentioned in Dave's post and by his commenters there, but here are a few more that come to mind for me.

I think the richly bizarre Coen Brothers movie Barton Fink would apply, depending on how you interpret the ending.  Has Barton gone insane?  Is he in Hell?  Or is the whole whole movie meant to be metaphorical?

One that certainly would apply is What About Bob?, in which Richard Dreyfuss plays an egotistical psychiatrist driven completely insane by a patient who intrudes upon the doctor's family vacation... Bill Murray is funny as the clingy patient, but for me Dreyfuss makes the movie.

Session 9 might also apply... it becomes clear by the end that one of the characters is insane, but you aren't quite sure when he became insane.  It may have started before the movie began, or it may have developed as a result of stress and remorse.  There also may have been a supernatural element to the madness, which confuses the issue a bit.