Halo 2

Foundation multi-player map:
Successfully complete the game in single player mode under any difficulty setting.
CrimsonReLLiK, Benjamin Marquez, RiCe1o5, Jeremy Bullard, Dave McMahan, Sturn36, and TNT.

Note: The following glitch may prevent some people from unlocking the map as it was originally intended. First, complete single-player mode under any difficulty setting. Create a new profile. Begin a co-op mode game on the last level with the newly-created profile as the second player. When you reach a section where you fly a Banshee, the new profile must perform a barrel roll and a loop-the-loop. Finish the campaign and the new profile can access the Foundation map when hosting split-screen, System Link, or Xbox Live games.
Command Station.

Alternately, if you have Xbox Live go to the Microsoft Content download menu and download all recently updated content. Go to a game menu. Go to change the options then change the map. It should be at the very bottom.
mikey smith.

Play the game all the way through under any difficulty setting. Throughout the game, there are hints such as "Turn on flashlight", "Go camo with the White button", "Barrel roll with the A button in the Banshee", etc. Do not perform one of these actions (preferably the flashlight or barrel roll) until the very last level of the game. Use it there and after you finish the game the Foundation level should be unlocked on your list of multi-player maps.
WangMan.

Unlimited ammunition:
Obtain either a battle rifle or a shotgun. Shoot yourself in the foot until you die. Then, dual wield SMGs, and repeat. If done correctly about four or five times, you should no longer have to reload, and have unlimited ammunition.
Martin Morales.

Hidden DVD art gallery:
Insert the Halo 2 Bonus DVD from the Collectors Edition. Hold Left at the main menu until the art gallery appears.
yrpaysi.

Hidden DVD message:
Insert the Halo 2 Bonus DVD from the Collectors Edition. Remain idle at the main menu. The Marine will begin to threaten you very comically for not starting the game. He will go through three different routines before it repeats.
PugDaddyD.