I don't know if mods count, but there was a Half-Life 1 mod called Afraid of Monsters. It was basically about a guy addicted to painkillers who starts hallucinating (or is he?) that he is in a nightmare world. Even with dated graphics, the monsters were creepy as hell, and the game frequently made you go "Oh Jesus no." The author used cheap tricks like turning out all lights in a level or creating invisible enemies you could only "see" with your flashlight. Playing it from start to finish, all after dark with the lights out, was an unforgettable experience.