Spotted in the Halo 3 credits…

Lord Hood
Ron Perlman (Hellboy)

Prophet of Truth
Terrence Stamp (Superman, Priscilla)

Sergeant
Nathan Fillion (Firefly, tight pants.)

Marines
Adam Baldwin (Firefly, Angel, not a real Baldwin)
Alan Tudyk (Firefly, Steve the Pirate)
Katee Sackhoff (Battlestar Galactica, might be a Cylon)

Brute Chieftan
John DiMaggio (Futurama, Gears of War)