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It was either Doom 3 or Quake 4, I can't remember.

When I was in school several years ago I used to have a habit of playing video games at all hours of the night. So there I was, playing at like 3 in the morning with all the lights off and there was a noise that sounded as if it were behind me. I turned around and looked behind me and of course there was nothing there, so I kept playing... took what had to have been 5 or 6 steps forward around a corner and this ungodly monster jumped out at me.

Scared the life out of me, that's for sure.
 
I would also suggest the Penumbra collection from Frictional Games (the developers of Amnesia). These games are very similar to Amnesia, are quite cheap ($10), and run very well on Macs.

They also scare the daylights out of players.
 
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