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.

Infinte 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.

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!!

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.

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.