The Exorcist is still the scariest movie I've ever seen. I love it, but I have yet to be able to view the entire film in a single sitting.
Also:
Blair Witch Project
Cloverfield
Burnt Offerings (terrified me as a child)
Night of the Living Dead (original)
The Omen (original)
Salem's Lot (original)
The Ring (original and American)
And though it is not out-and-out "scary", one of my all-time favorites in the horror genre is A Tale of Two Sisters. The American "remake", entitled The Uninvited, strays too far from the original story and it didn't capture the atmosphere or devastating effect of the original.
And RobbieJ, you hit the nail on the head with the Albino from Foul Play. I remember having nightmares about him when that movie was originally released. Still love that movie, though. Though I know it sounds dopey, I often thought it could make a really fun musical.
"What can you expect from a bunch of seitan worshippers?" - Reginald Tresilian