Halo 2 Limited Collector's Edition
From Halopedia, the Halo wiki
Limited Collector's Edition Disc
The Limited Collector's Edition Disc provides documentaries on the development of Halo 2 and the Halo universe as a whole. It also provides access to deleted scenes and other content from the cutting room floor.
On the main menu of the DVD where it shows behind the scenes, art gallery, etc, do not do anything. Just simply wait. Soon, you can hear a voice of a soldier begging you to press play.
On any section of the DVD(NOT watching the movie, just the sections), press and hold the left button on your DVD remote. It should cycle quickly through the selections of the section and soon, it will send you into a secret art gallery. Each section gives a different gallery.
Limited Collector's Edition Manual
The Limited Collector's Edition provides an exclusive game manual, which features a Covenant Elite Zealot on the manual's cover instead of the standard edition's Master Chief cover. The manual is written as if it were a report made by the Covenant Supreme Commander of the Fleet of Particular Justice to the Prophet of Truth, after the Battle of Installation 04.
Conversations from the Universe
The Conversations from the Universe is a booklet containing various conversations, letters and text logs from characters in the Halo universe.