My personal scariest moment comes courtesy of Rebellion Software, circa 1994. The original Alien vs. Predator on the Atari Jaguar was one of the first shooters I had ever played (along with Marathon, but that wasn't very scary). I recall creeping around the empty corridors of the Space Marine base with the ambient sounds of machinery in the background, and no warning that you're about to have company. Suddenly, you turn around, hear that familiar screech, and see a pack of Aliens approaching -- which really sucked when I was entering a turbolift and had nowhere to run. The Predator's trademark rattling sound also scared the bejeezus out of me because you never knew when he was going to decloak and start whaling on you.
Let's hear your scariest moments.