Alien Vs. Predator was such a great game, although the graphics are a bit dated now. I still play the Jaguar version from time to time. The Alien's ability to lay eggs and sneak through the ducts, the Predators cool weapons and special abilities like thermal vision, the feeling of terror you get as you play as the Marine (until you get the flamethrower which helps a little bit). Great stuff. Also check out System Shock 1 & 2.