That damned wallpaper!
I've skimmed this messageboard but can't find reference to what I consider probably one of THE most unsettling scenes I've ever witnessed on screen: The camera closing in and out on that raised/textured plaster/wallpaper during the night. Is that a hole in the paper? Why does it look like some twisted, evil lion's face? The changing light and creeping shadow! That horrible mumbling! The subtle rotation of the camera!(Let alone Eleanor's final realisation about her hand being squeezed!!) No CGI can ever match the skin-crawling unease of those few moments.
Despite first seeing the film when I was a young teenager, that scene has lodged itself indelibly in my head and continues to make the hairs on my neck prickle whenever I watch it to this day...
(Addition 15/12/09: The power of imagination never dies! This was used to good effect in the recent 'Paranormal Acitivy' - in fact the loud banging on the door must surely have been inspired by 'The Haunting'!)