Halo 3: ODST Easter Egg - Credits on Oni Alpha Site

Just play through Oni Alpha Site as you normally would, when you get to the part where you defend the inside of the building you will see a pile of sandbags (not by the machinegun turret). Go past the sand bags you will see a whole bunch of pillars, you will also see a wall with a whole bunch of names on it, those are all the creators of Halo!