I've played a lot of horror games in my day, and while P.T. takes the cake for creepiest experience ever, Amnesia: The Dark Descent is hands down the scariest full-length game.
So it's a bit of a surprise that it's being given out for free right now. Courtesy of Humble Bundle, not only do you get Amnesia: The Dark Descent but also Amnesia: A Machine For Pigs. That's over 25 hours of gameplay, no strings attached.
Most people who play Amnesia have never bothered finishing due to its overwhelming psychological horror. If you're like me, then you see that as an invitation to a challenge. If you're like the guy in the video below, then that'll mean an invitation to higher blood pressure.