F.E.A.R of course! Also, Condemned. I know it was for Xbox as well, but I think it had a creepier effect on PC. Any scary game does, in my opinion. Aliens Vs. Predator 2 is also great, for it's single player, but also it's intense multiplayer. Diablo 2 is good too, and Black & White is just awesome! If you haven't heard of that, it's a real-time strategy/ rpg game where you get to be a god and raise a creature and everything. You can choose to be good, and the people will love and worship you, or you can be evil, and people will fear and worship you.