The 'Face in Halo is a easter egg that is in Halo 2 on the multiplayer levels Beaver Creek and Containment, where a face can be seen in the Halo ring. The face is thought to be Bungie team member Frank O'Connor(or as he is usually called, Frankie) or Chris Carney.
On Containment it can be found by standing near the red team's base where the turret is, then zooming in with a sniper rifle at the ring (while a normal zoom can reveal the face, the sniper rifle is the ideal zooming tool).