On any Halo 2 Anniversary map, load up an Assault game mode. After that, pick up the bomb and throw it off the map, and keep repeating until there is a new message on the top of the bomb, it should be a Spartan Mark VI helmet, a + sign, and an outline of Texas with a star on Austin, and the result is a heart.
Halo 2: Angry Mode
When you are the Master Chief trying to recover the index from the Prophet of Truth on the Covenant Holy City go until you reach the first outdoor area with dirt and vegetation. Take a right as you enter this room and run up to the wall past the gravity lift that leads up to the sniper point. Get on top of the wall by going up the dirt mound on your left. Follow the wall until you reach the floating skull called "Angry". This skull makes enemies fire at a faster rate.
Halo 2: Hold 3 Pistols
First Go to a level that you can get three pistols on, such as outskirts. Go pick up a pistol. Then swith to your second weapon and pick up dual weapon, such as two smg's. Go to another pistol and press x to pick it up. Now you have a pistol in you secondary and first slots. Now you can go pick up a third pistol and set it to your left hand(Press Y to do this). You can get up to 144 AMMO! with three 96 ammo with the two and just 49 with one.
Halo 2: Infinite Camo
After getting the envy skull go through a checkpoint while in active cammo mode. To be safe use "reset to last checkpoint" so you can check if you did it right. Save and quit; then turn off the xbox to remove the envy mode. When you reload your file you will find yourself with unlimited camo.
Halo 2: Multiplayer Map- Foundation via Xbox Live
Bungie has decided to give the multiplayer map to anyone who signs on via Xbox Live. You will be prompted with an update as soon as you try to sign on. The update will unlock the map, improve support for 480p resolution display's, and speed up wait times in matchmaking.
Halo 2: Overloading objects on the map
It is possible to overload certain objects in many of the Halo 2 Multiplayer Maps. By overloading I mean removing it from the map altogether. You can overload most anything that is not actually connected to the map itself. Please note that it seems that you will need at least 8 others for this to work. The easiest example is in Zanzibar. There is a bridge that you can knock down to easily get to the sword on the large spinning wheel. Knock the bridge down and run to the end where it is squared off. You and all your teammates should bunch up as close as possible in the corner and look straight down at your feet. Make sure everyone is looking down and the bridge should overload and disappear. It will be gone for the remainder of the game/round. You can also use this overloading glitch on Midship(on the floating platform) and Headlong(there are two places in Headlong...the hanging construction beam and the hanging platform right next to the beam....however you may need around 12 others to overload that platform). These are the ones I have seen myself but there may be others in other maps so good luck hunting!
Halo 2: Skip tough Brute battle before the end boss
When you are flying around in the Banshee during the final level when you are giving support to your ally. You can skip directly to the final boss sequence if you are still in the Banshee after te door gets blown open. Fly to the right of the control centre building and aim towards the bottom right and if you fly close enough you should get a white screen indicating that you have started the cutscene. When it's over you will be at the final boss without fighting your way to him.
Halo 2: Unlimited Sword Ammo
You can do this in any two-part Campaign missions (i.e. Outskirts and Metropolis). If you finish the first level of the two-part mission with a sword with no ammo, you'll start the next level with a sword that has unlimited ammo. Though the sword doesn't show up when you arm it, it still makes all of the sound effects and acts exactly like a normal sword.
Par Score requirements
Complete the following missions with a score that is more than the indicated values. Activate as many scoring sculls as possible, and play under the Heroic or Legendary difficulty setting. You can also increase your score through melee attacks, headshots, killing sprees, and scoring multiple kills.
Halo 2
Cairo Station: 25,000
Delta Halo: 25,000
Gravemind: 25,000
High Charity: 25,000
Metropolis: 25,000
Outskirts: 25,000
Quarantine Zone: 25,000
Regret: 25,000
Sacred Icon: 25,000
The Arbiter: 25,000
The Great Journey: 25,000
The Oracle: 25,000
Uprising: 25,000
Halo 3
Cortana: 25,000
Crow's Nest: 25,000
Floodgate: 25,000
Halo: 25,000
Sierra 117: 25,000
The Ark: 25,000
The Covenant: 25,000
The Storm: 25,000
Tvaso Highway: 25,000
Complete the following missions within the allotted time. This can be done playing under the easy difficulty setting without activating any Skull Modifiers, and skipping through unnecessary engagements.
Halo 1
Pillar of Autumn: 15:00 minutes
Halo: 20:00 minutes
Truth and Reconciliation: 20:00 minutes
Silent Cartographer: 15:00 minutes
Assault on the Control Room: 15:00 minutes
343 Guilty Spark: 15:00 minutes
The Library: 25:00 minutes
Two Betrayals: 20:00 minutes
Keyes: 15:00 minutes
The Maw: 15:00 minutes
Halo 2
Cairo Station: 15:00 minutes
Outskirts: 15:00 minutes
Metropolis: 15:00 minutes
The Arbiter: 15:00 minutes
The Oracle: 25:00 minutes
Delta Halo: 15:00 minutes
Regret: 15:00 minutes
Sacred Icon: 15:00 minutes
Quarantine Zone: 15:00 minutes
Gravemind: 20:00 minutes
Uprising: 15:00 minutes
High Charity: 15:00 minutes
The Great Journey: 15:00 minutes
Halo 3
Sierra 117: 15:00 minutes
Crow's Nest: 20:00 minutes
Tvaso Highway: 20:00 minutes
The Storm: 15:00 minutes
Floodgate: 15:00 minutes
The Ark: 20:00 minutes
The Covenant: 20:00 minutes
Cortana: 15:00 minutes
Halo: 20:00 minutes
If you play as the Arbiter in Halo 2 Anniversary. Use Cloak. Then switch to classic mode and wait till you can cloak there as well. Then switch back to Halo 2 Anniversary and voila, permanent cloak.