These cheats are ONLY for the Custom Edition of Halo: Combat Evolved. Open the properties window of the Halo shortcut and add ” -console -devmode” to the end of the target field. Apply changes and start the game using that shortcut. Press [~] during game play to display the console window, then enter one of the following codes to activate the corresponding cheat function (codes do not work in online games):

cheat_medusa <0 or 1> - Instant deaths for enemies that see player
speed <0-20> - Set game speed game
cls - Clear console window
cheat_bottomless_clip <0 or 1> - Unlimited ammo and no overheating for weapons
cheat_infinite_ammo <0 or 1> - Unlimited ammunition
cheat_super_jump <0 or 1> - Super jumps
cheat_bump_possession <0 or 1> - Possess touched character
cheat_deathless_player <0 or 1> - Invincibility
cheat_spawn_warthog - Spawn Warthog
cheat_all_powerups - Spawn all power-ups
cheat_all_vehicles - Spawn all vehicles
cheat_all_weapons - Spawn all weapons
cheat_teleport_to_camera - Teleport player to current camera position
quit - Quit game

Get Alternate Ending:
After final cut scene, wait for the credits to finish to watch yet another cut scene. If you are playing on Legendary difficulty, there is yet another (but small) cut scene after the one after the credits. (Two total after the credits if on Legendary difficulty).

Get Katana:
Unlock all achievements and collect all skulls.

Unlock Multiplayer Armor Parts:
Complete the indicated task in Campaign mode to unlock the corresponding armor part in Multi-player mode.

Spartan Security Shoulders: Earn 250 Gamerscore points.
Spartan Security Head: Earn 1,000 Gamerscore points.
Spartan Scout Shoulders: Earn the “We’re in for Some Chop” achievement.
Spartan Scout Head: Earn the “Used Car Salesman” achievement.
Spartan Scout Body: Earn the “Too Close to the Sun” achievement.
Spartan Rogue Head: Earn the “Spartan Officer” achievement.
Spartan ODST Head: Earn the “Spartan Graduate” achievement.
Spartan Mark V Head: Earn the “UNSC Spartan” achievement.
Spartan EVA Shoulders: Complete “The Ark” under the Normal or higher difficulty.
Spartan EVA Head: Complete Campaign mode under the Normal or higher difficulty.
Spartan EVA Body: Complete “Tsavo Highway” under the Normal or higher difficulty.
Spartan EOD Shoulders: Complete “The Ark” under the Legendary difficulty.
Spartan EOD Head: Complete Campaign mode under the Legendary difficulty.
Spartan EOD Body: Complete “Tsavo Highway” under the Legendary difficulty.
Hayabusa Shoulders: Collect nine hidden skulls.
Hayabusa Helmet: Collect thirteen hidden skulls.
Hayabusa Chest: Collect six hidden skulls.
Elite Flight Shoulders: Complete “The Ark” under the Heroic or higher difficulty.
Elite Flight Head: Complete Campaign mode under the Heroic difficulty.
Elite Flight Body: Complete “Tsavo Highway” under the Heroic or Legendary difficulty.
Elite Commando Shoulders: Earn the “Killing Frenzy” achievement.
Elite Commando Head: Earn the “Overkill” achievement.
Elite Commando Body: Earn the “Triple Kill” achievement.
Elite Ascetic Shoulders: Earn the “Overkill” achievement.
Elite Ascetic Head: Earn the “Steppin’ Razor” achievement.
Elite Ascetic Body: Earn the “Up Close and Personal” achievement.

Use Anti-Air Wraith:
Requirements: Battle Rifle/Carbine and lots of patience.
All you have to do is eliminate the secondary gunner and then stand on top of the driver. Bash open the hood (melee) until you can see the driver. Then melee the Brute until the helmet is gone (or if the helmet is not on the Brute, then do not melee).

Now comes the hard part. You have to find a spot where it DOES NOT give you the option to board the enemy Wraith (Hold RB…) So for the split second that this does not show, hold RB and shoot the Brute in the head (while still holding RB). You should now board the Wraith once the Brute is dead.

If you were not holding RB while you killed the Brute, the Wraith would most likely explode.

Have Fun with these Halo 3 Cheat codes for X-BOX 360
Also, if you get out of the Wraith, it will explode, not allowing you to enter it again.