I love foggy films. They almost belong in a sub-genre all their own.
THE MAN FROM PLANET X is perhaps one of the foggiest movies ever.
Here are a few off the top of my head:
STRANGLER OF THE SWAMP
PLANET OF THE VAMPIRES
THE SLIME PEOPLE
THE MAN FROM PLANET X
CITY OF THE DEAD (aka HORROR HOTEL)
THE FOG (of course)
Can you name any other fog bound horrors, mysteries. or spook shows?