I'm pretty sure almost everyone has played a game thats scared the crap out of them so much that they can't stop thinking about it.
I have played one such game and it was called Shivers.
Its an old horror adventure game by Sierra back when graphics weren't that important.
Man, that game really scared me.
The fact that you are dared to spend a night in a derlict museum for the strange things in the world is so creepy.
Here's the intro for the game:
http://www.youtube.com/watch?v=ynwzw7L BMjo
It also had a sequel by the name of Shivers 2: Harvest of Souls, it was quite like the first game expect it took place in a ghost town by the name of Cyclone.
gamer30hrplus
Toss up between Silent hill and Dino Crisis. For series it would be Resident Evil.