Shogun 2 is really a good turn based strategy game.
Half Life 1 (not really graphically intensive, but a great game)
Metro 2033 (could cripple a good high end system, with all the eye candy turned up)
Left 4 Dead 1, 2
Mass Effect 1, 2, 3
The Elder Scrolls: Oblivion, Skyrim
Deus Ex: Human Revolution