According to Bungie, Halo 3 will be released in 3 flavors.
Here's the breakdown for each edition and remember, Bungie stresses that all of these items are subject to change or replacement.
Regular Vanilla Edition:
Just Halo 3 for Xbox 360 and a manual.
Halo 3 Collector's Edition:
Collectible Metal Case
Halo 3 for Xbox 360
Bestiarum: A hard cover, bound collection of information and art covering the species, cultures and civilizations of Halo 3.
Interactive Xbox 360 Bonus disc
Halo 3 Legendary Edition:
Fully sculpted Spartan helmet: An accurate model of the Spartan Mjolnir Mark VI helmet.
Halo 3 for Xbox 360
Storyboard Collection: Exclusive selection of storyboard art from Halo 3's cinematics.
Legendary Disc: a DVD disc that will play in your 360 or on any DVD player and features a wealth of rich, unique and exclusive content.
Interactive Xbox 360 Bonus disc
The interactive Xbox 360 Bonus Disc available with both the Collector's and Legendry versions will include:
Hi-Def Halo: Halo-themed calibration tool to make the most of your Home Theater experience.
Art Attack: Exclusive Halo 3 Gamer Pics and Dashboard theme, a vast gallery of concept and production art (complete with musical accompaniment) and a fully playable version of a classic Halo-themed mini-game.
Featurettes: A series of short films covering a range of subjects, from making your Network play nice with Multiplayer, to the challenges of building the biggest Halo ever.
The Legendary Disc, exclusive to the Legendry version, contains:
Halo Cinematics Reloaded - completely remastered, hi-res versions of the Halo and Halo 2 cinematics with 5.1 audio and complete with optional "Director's Commentary" track featuring Marty O'Donnell, Joseph Staten and Jason Jones.
Bestiarum - an expanded digital guide to the creatures, cultures and worlds of Halo 3, with art and assets exclusive to this interactive edition.
The Cortana Chronicles - Jen Taylor, the voice of Cortana, takes us on a tour of the worldwide Halo fan community ,where she meets Halo fanatics from near and far, obscure and famous alike.
Plus: All-new, exclusive Machinima episodes from Red Vs. Blue and This Spartan Life and a novel "mockumentary" voiced by Sgt. Johnson himself, David Scully.