Diabolique is a French black and white thriller/horror film from 1955. If you like Hitchcock movies, you'll love it. The plot goes like this: A cruel and abusive headmaster at a boys' boarding school has married his frail wife for her money and property (including the school). He's having an affair with one of the teachers, the fantastic Simone Signoret in black sunglasses. However, he treats them both badly, and they band together with their plot to kill him. After the murder, his body vanishes, and paranoia ensues.
Diabolique is one of the greatest suspense movies of all time, and, if the ending doesn't get you the first time you see it, you ought to check yourself for a pulse. Yes, it's in French and subtitled, but the plot rolls along with such building dread that you won't mind it. Also, Hitchcock wanted to buy the rights to make this movie, but he missed his chance by just a few hours. I doubt he could have done a better job. It was one of the first movies with a spoiler note, too, asking the viewer not to ruin the ending by telling anyone. Genius.
Apparently, there was a remake in the nineties with Sharon Stone. From what I've read, it's not worth your time. This one though will amaze you.