The adventure game 5 Days a Stranger had some pretty scary stuff about 1/2 of the way through and the atmosphere keeps building from there; I remember being spooked by some of the scenes, which is impressive considering it is a point and click adventure game. If you check it out, be sure to have the full sound and music on, as that really added to the atmosphere.
7 Days a Skeptic (the first game's sequel) also was somewhat scary, although is more like the movie Alien.
You can get both of these games for free from the developer's site: link
I also found the intro room for Alone in the dark to be pretty scary. At the time I tried it (several years ago), I couldn't get my computer to get past the first room, from a technical standpoint, as I couldn't get the equipment to work and got eaten by monsters. Based on that (as surely the actual game is even worse), it was probably even scarier than the other games I quoted.
Also, there was some zombie/alien invasion survival 3D adventure game that I thought was quite scary and liked playing, but couldn't play more than 20 minutes without getting motion sickness; I'd recommend it, but can't remember the name as I didn't get far enough through it to learn much of the plot (it might have been Half Life).
Edit: the game I was trying to remember the name of is "Singularity." It was only the first part of the game that I played though, due to motion sickness, so it might not be scary later in the game. You can see a trailer for the game, here, which gives you an idea of the great atmospherics, though: link
In a related note, if you're looking for suggestions for scary games, when I was trying to find the title of the game I was looking for, I found this list of scary games that look like they might be good. I haven't played any of them, but it looks like a good list: link
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Last edited by dancing_penguin on 23 Sep 2011, 5:57 pm, edited 1 time in total.