The Effective Use of Cake in the Manipulation of the Human Psyche
In Portal, GLaDOS, the computer/life-form operating the Aperture Science Test Facility promises the protagonist, Chell, a cake should she complete all test chambers. GLaDOS is very obssessed with cake, probably due to her malfunctioning systems and seems to consider cake a highly-regarded reward for test subjects. As well as GLaDOS directly mentioning cake, a picture of cake appears on the wall at the end of test chamber 20, and numerous pictures of cake appear on the monitors of GLaDOS herself.
At the end of the game, in the final fight GLadOS's processing unit lists items for a cake recipe and in the famous song Still Alive, which plays in the credits of the game. Some of the lyrics mention cake and at one point in the credits GLaDOS displays an ASCII picture of a cake. One hidden room in the game also has the words "The Cake Is A Lie" etched in blood repeatedly down the walls and this phrase has become somewhat of a meme among players of the game. A cake does finally appear in Portal, just before the credits and is revealed to be hidden in a room deep within the Aperture Science Facility.
Achievements involving the advanced maps in Portal also feature cake in their name i.e. Cupcake, Fruitcake and Vanilla Crazy Cake. The constant references to cake have aided to fuel the humor in Portal and have become one of the things the game is well known for.