Penumbra: Black Plague!

I just noticed a rather interesting phenomenon in that game while playing it in the dark in the middle of the night; being a game that encourages you to run and hide from enemies, since you can't defend yourself whatsoever, the scariest part by far is sitting in a corner listening for any trace of whatever monstrosity you don't want to come face to face with.

See, when you do face it down, and it starts running towards you to carve you up, it's no longer as scary, because your mind isn't filling in the blanks anymore, and it's very difficult for the designers of the game to live up to the fright that your mind conjures up.

The best part, and by that I mean the very most terrifying, is when you just hear the monster, or spot it turning a corner, not entirely knowing what it is you are perceiving. There really should be more of that in survival horror games, and less of the old bog standard blowing everything up with a shotgun!