After defeating the end game boss in the level "Final Showdown", head under where you killed him (there should be two areas where you can step down into a cellar like area). Head through here until you reach a new passageway with a breakable wall at the end. Bust through the wall and you will enter the Id Gallery, a collection of photos of Quake II's designers. If you follow a ladder down even more, you can enter an unusual room filled with a Quake II enemy and some ... well, some friends.