Cake

In the last Test Chamber, as you are nearing the end, you approach an incineration room where GLaDOS assures you that you will be receiving cake soon. When you escape from the flames GLaDOS tries to trick you that if you were to surrender, you would be given cake. From this point on there's alot of blatant references to the cake being a trick with the famous "The cake is a lie!" statments.

Even at the very end during the credits, the cake is mentioned in the funny song that accompanies the credits.