This scene comes from my favorite Alfred Hitchcock film, and one of my favorite films in general. It is quite an unnerving scene because, for starters, it is killing off the woman who has been the main focus throughout the film. Usually in horror films, the main character is fine throughout the whole film and ends up being the only survivor. With this film, however, it is throwing you for a loop by letting you get to know this character and then suddenly taking her away.
The scene starts out perfectly normal with Marion Crane taking a show. After a while, though, you see a dark figure on the otherside of the shower curtain. This adds a bit of dramatic irony, because you know something is about to happen even though the main character does not.
One thing that makes this scene frightening is that it plays on how vulnerable we are in the shower. In the shower we are completely naked, standing on a slippery surface, and basically backed into a corner with nowhere to run. You see a lot of movies that use this including Cabin Fever and The Grudge.
*Little factoid: I just realized that the two scenes I picked so far for my Iconic Horror Scene column are from films inspired by Ed Gein, a serial killer from Wisconsin, which is the state where I grew up. So.....go Wisconsin?