The face similar to the one in Containment.
The Face on the Moon is located in the Halo 3 multiplayer map Standoff. On the moon is the face of Chris Carney, a Bungie employee.
One way to view it is to zoom in on it using a Sniper Rifle. You can also use the Pan Cam out-of-the-map trick in Theater mode to get very close to the moon.