System Shock 2 was so freaky I couldn't play it. All those weird freaks groaning and coming from behind while you try to deal with the automated machine guns... AvP2 aged me 2 years easy. Undying nearly killed me. Still weren't as bad as SS2.
The Thing would be pretty freaky if it had friendly guys aka half-life that followed you around until all of a sudden... groan...stretch..creak..mmmUUHHHHH!!!!!..