Earth Impact: There is an arcade machine on the Reliance level in the right hand building on the ground floor.
X-Space '92: In the Madness level there is a hidden floppy disc which you can use on any computer on the level to play (the nearest computer to the disk is the reception computer). To get to the disc you have to go just past the reception desk section then through to the right and in the center of the corridoors with all the mental patients there is a room. Inside the room is the shotgun and on top of one of the shelves is the disc. You will have to use Telekinesis to reach it.
The high scores of both of the above games are tracked in your statistics menu.