Uhm...
The Zero / Fatal Frame / Project Zero games, all of which you can emulate perfectly on any modern PC. The games usually follow a girl or group of girls exploring someplace haunted and related to japanese folk beliefs with no more weapons than a special camera. Fatal Frame 1 is short, rough around the edges, very atmospheric, and has some brutally hardcore combat. Fatal Frame 2 is a bit longer, much easier, incredibly atmospheric, and probably the scariest game you will ever play. Fatal Frame 3 is very long, can have you wander for hours if you don't pay attention to the little clues that tell you what to look for and where to do so, is way harder than the second game while not reaching the levels of the first one, and has a couple of really creepy and disturbing sequences (Sure, Miku, CRAWLING UNDER THE FUCKING FLOOR sounds like a good plan.). I have yet to play the fourth one.
Special mention goes to the ghosts you face, most of which are pretty disturbing and have little implied-only backstories that rule their behaviour and appearance, and how thick the atmosphere is: Play them at night, alone, with headphones to lose your ability to sleep with the lights off for days. The puzzles are simple and straightforward, and usually take the form of either doors sealed by certain items or presences about which you only get vague information about in the form of a ghostly photograph you get by taking a shot at the seal and must then look for, or self-contained puzzles like those in games like Shivers.
Combat is mostly pattern recognition, timing, and cold blood (as it rewards being patient, keeping the enemy in focus as long as possible, and waiting for the very last moment before being touched to take a pic). The game plays in third person for exploration but goes into first person when you prepare your camera, and there are more than a couple of WTF moments when you take your camera out just because only to have something react to it unexpectedly and give you a beautifully subtle (or not so subtle) first person scare, or maybe you just notice some small detail that was hard to notice in third person mode and all of a sudden you want to hide because you are SURE SOMETHING HORRIBLE IS GOING TO HAPPEN THE MOMENT YOU TAKE ANOTHER STEP.
Finally, all games default to the BAD ending the first time you play them. You must play them on hard mode (and in some games complete an optional quest that is only activated during a second playthrough and whose existence you have to notice on your own from the reading materials you collect), unlocked after clearing the game in normal mode, to get the GOOD one. Canonical endings are the BAD ones for the first two games, and still unknown for the third and fourth. Contrary to most other games, though, atmosphere is so thick the games don't really become less scary if you replay, and at times is probably the opposite. Oh, and most ghost encounters are random (i.e: you enter this room, nothing attacks you. Next playthrough you enter this room, The Wandering Monk appears and serves you your butt), and certain boss fights change depending on variables no one is sure to have mapped.
In any case those are some of the few games that come close to completely paralyze me because I know something horrible is going to happen and I don't want to see because then I will not be able to sleep ever again. And for the record I may not be the bravest person to have ever lived but the second game managed to once completely paralyze me out of fear DURING THE DAY, to the point of making me physically ill for an hour or so. Fuckin' Kiryu twins.
Give also a try to Ju-On and The Calling for the Wii, though again both can be easily emulated on modern hardware. They are the same kind of games the Fatal Frame ones are: Asian Horror impecably translated into videogame form, you being a pretty much defenseless girl or boy in a very haunted place that wants you dead from a heart attack oh-so-very-much. They are both exploration heavy and first person, and very disturbing. And just like Fatal Frame they are not much into "BOO!", instead building a stupidly creepy atmosphere.
Then there's the Forbidden Siren games, a pretty weird mixture of sneaking puzzler (think commandos) with horror by some of the guys behind Silent Hill, though it is more unerving than scary: It's the mixture of creepy and OH-GOD-IM-GOING-TO-BE-FOUND that gets you, the game is very tense but it will not make you vomit your heart like some of the others I mentioned will. It has some pretty cool mechanics, too, in that your main tool to study the enemies' routes, recognize their vision cones, and scout the levels is to sight-jack them. Basically, you see through their eyes. Which, of course, gave them the idea of making the only character who can move while sight-jacking BLIND. Thank you, game.
It's hardcore and oh-so-frustrating.
And since both the Clocktower games and Haunting Ground were already mentioned I'll add White Day and Hellnight to the list, too. The first is a korean horror PC game that has an english patch, the second is a localized playstation one Japanese game. They are the same kind of game as Clocktower and Haunting Ground but in first person: You explore huge places and solve puzzles while trying not to get found, or caught when found, by something implacable and very killy. Hellnight is atmospheric but not very scary, White Day is one of the most disturbing games you will ever play.
That's about it. I'll post again if I remember anything else.
