I know this is probably going to sound dumb, but...
Easter eggs? XD;
Edit: Oh wait! I have found something I can mention! In the Collective Unconcious, floating somewhere behind Oleander's Basic Braining is a slide that looks like one of the credits from the opening, but it reads "YOUR NAME HERE". You can levitate/float to it and look in first person view at it. It's blue. I'd show pictures, but my game is on PS2