Oddly enough, the only PC game that has actually managed to scare me recently isn't a big budget FPS horror game, it isn't even a SMALL budget game. It was a free-to-download adventure game called '5 days a stranger'. It's very retro, along the lines of the original monkey island games, but with a horror instead of comic theme.

But despite all that, it REALLY knows how to press the tension buttons.

Check it out

Yes, It was called StoneKeep....a RPG in a bottom of the a castle....there was a part where you go into the sewers and you could hear these giant snakes coming for you in the water.

System Shock 2 was pretty good at scaring the bajeebas out of me.

Oh, and the Longest Journey 2 was quite creepy in a number of places at the start of the game.

I don't think I played them directly, but 7th Guest and 11th Hour (they're both technically of the same continuity).. Those games were freaky. And the puzzles seemed insane.

Can't play 'em. For some reason I always get a headache and feel nauseated when I try to play a video game...

Pong.

Those bitmapped cubes give me nightmares...

Fear
System Shock 2
Doom 3

System Shock has come out a couple of times...gonna have to give that of a shot.

For me , playing the original Half-Life in a dark room late at night. Those headcrabs leaping outta nowhere gave me a few "skid mark" moments believe me ...LOL!  

Heh. This was a while ago. Quake II. Sidling down a ramp in a darkened area, concentrating on finding the power cube receptacle. Silence but for my labored breathing and the faint thump of distant machinery. I'm down to 15 hit points. I spot the hatch cover I'm looking for and move out into the chamber, away from the wall. Suddenly, from both above and behind, the sound of iron maidens rising from the floor. You know the one:

"Hhhhhuh... OOOooooohhhh!"

Then the sight of missiles fired at short range; and I couldn't even react because I'd thrown the freaking mouse at the speakers!

It was a free-to-download adventure game called '5 days a stranger'. It's very retro, along the lines of the original monkey island games, but with a horror instead of comic theme.

If you liked that one you should play '7 days a skeptik'. Its a sequel to '5 days a stranger' and it is also possible to download it for free. Trust me it scares the hell out of you.

Almost forgot another very tense,scary game ... "Aliens vs. Predator 2"! Most of the time your surroundings are very dark, with only your little lamp (which only lasts so long before it goes out)to guide you, and all the time hearing distant screams and screaching and the "thump,thump,thump", of the motion detector as the Aliens chase you in the darkness ... "white knuckles" all the way!

AVP 1 & AVP 2
FEAR
Doom 3 (all three version made me physically sick while playing.)
System Shock 1 & 2
Far Cry (some parts)
Half Life 2 (sewer part when zombies jump out.)

Clive Barker's Undying was a good scare for it's time. It had excellent use of sound effects to add to the atmosphere.
F.E.A.R. (of course)

FEAR
But Far Cry? It's not scary, it's just really hard to kill those big monkey things.