Horror games can be a tough sell, because it's hard to tell someone that a game is sooo scary without raising their expectations so high that they'll never actually be scared. Thus, I'll try to stick to the facts when I talk about Amnesia: Dark Descent.
It's out on September 8th, and was developed by Frictional, who also made the Penumbra games, which were scary enough to be included in the Humble Indie Bundle. It's a "survival horror" game in a truer sense of the word than has been applied to the likes of Resident Evil and Dead Space, which of late have been more about the shooting-dudes stuff than the being-scared-out-of-your-wits stuff.
It's also got a demo, which you can download from many sources here. It'll be available on most of the digital distribution-type places, and preordering gets you a discount off the already-low price of $19.99. And it's available for PC, Mac, and Linux.
So go. Try it and get it. Or check out a trailer below. Low risk to everything but your sanity. Mwahahahaha!