
Diabolique

The cruel and abusive headmaster of a boarding school, Michel Delassalle, is murdered by an unlikely duo -- his meek wife and the mistress he brazenly flaunts. The women become increasingly unhinged by a series of odd occurrences after Delassalle's corpse mysteriously disappears.
Finally watching it again after 10 years and oh, man, this is still one of the best horror movies I've ever seen in my life.
The tension just never lets up, Clouzot knows exactly how to leave us on the edge of our seat with the most simple and apparently insignificant details...
[spoiler](And that ending, my friends... that ending...)[/spoiler]