Here's a small example of some cheats/easter eggs in the game
-Loading a certain filename
--e.g. Loading a file named "thinkwithportals" will make Portal music play in the background.
-Console commands (duh)
--e.g. Certain command that will make enemies stop appearing.
-Interactive Environment
--e.g. You put "turkey" into a keypad and you get five levels up.
Those won't necessarily be in the game, but those are some examples.