The Night of the Hunter (1955),
This is one of the creepiest movies you'll ever see! I'm surprised it's not often mentioned in the same breath as Psycho. Heck, I'll go one step further and say it's better than Psycho. Every scene is meticulously shot, with great use of shadows and interesting angles (some with German-expressionism influence). The scene with Robert Mitchum explaining his tattooed knuckles was later lifted by Spike Lee for his movie Do the Right Thing. I don't want to give away too much of the story, but let's just say it's like every bad dream you've ever had.