Tetris is one of the few people have mentioned which I think deserve to be on that list. Many others deserve to be on a different list (games which have changed gaming). I could think up a pretty extensive list of influential FPS games (influential on other FPSs, not on the world). Wolfenstein 3D, Doom, Duke Nukem 3D, Half-Life (and maybe 2), Deus Ex, Thief: The Dark Project (and 2), System Shock (and 2)...