Cheat Codes:
yeah yeah yeah...... i know there's no "cheats" released for halo yet, but there are some pretty cool hints listed here, and glitchs in the game, so enjoy!

Alternate ending sequence - Successfully complete the game in campaign mode on the legendary difficulty setting to view a different and longer ending sequence.

HINTS:

Hint: Defeating Flood: In the later levels of the game you will encounter new enemies called the Flood. These enemies will be small at first, but they will attack and control anything alive. Use a shotgun on these baby Floods as they tend to group in large numbers. By using the shotgun it will send a storm of bullets and kill most of them that are around. Use a shotgun on the possessed Flood, and it will kill most of them in one shot. This is made easier by the fact that most of them carry shotguns.

Hint: Defeating Grunts: When the Grunts are sleeping you can beat them to death with the butt of your gun without waking up any of the other Grunts. You can usually kill a four to seven of them before an alien spots you. They start sleeping in level 5, Assault on the Control Room. Just remember not to shoot or they will wake up.

Hint: Defeating Jackels: Use a photon weapon to destroy a Jackel's shield quickly.

When you encounter a Jackel, run up and hit it with your weapon or get out a gun that uses bullets, not energy.

Hint: Covenant dropship: When Covenant dropship is getting ready to unload a group of Covenants, throw a grenade into the door as it opens. The blast will kill several of the enemies since they are all in one area.

Hint: Assault On The Control Room: Spinning ring: At the Halo start up screen, choose multiplayer mode. Choose cooperative play at the next screen, then choose your profiles at the next. Choose the level where in the beginning it says, "I would have been your daddy" (Assault On The Control Room). On the difficulty screen, choose legendary. When the level starts after the introduction sequence, have one of the players quickly press X when the "Press X to get on the Pelican" message appears. That player will re-board the ship and start flying with it down the hole. The other player will quickly die because of the aliens, and will be respawned on the spinning ring. Make sure to jump where there are gaps. If you are good enough, you can even jump to a nearby ledge. Additionally, once you are respawned on the ring, quickly jump off and fall to your death. Sometimes you will be respawned on the lowest ledge in all the blue haze, and a "Press X to flip the Pelican" will appear that the top of the screen very briefly.

Hint: Halo: Fast kills: During level 2 "Halo", you will hear the warning "Take cover -- Covent ship is in boundaries". Run into the ship that you landed in and wait for the enemies to appear. Your motion detector will indicate when they are present. They will eventually walk by, not noticing you. Throw a grenade and start shooting to kill them much faster.

Hint: The Maw: Grunt message: There is a Grunt in the level that will give you a message. To get to him, go all the way past where the drop-ship is going to pick you up. About three rooms after that, there will be a shortcut to your right. Go in it. In the middle will be two doors, to your left and right. Unlike the shortcuts, the door to your right will be open. Enter it. At the end of the hallway, get out of the jeep and the Grunt will be there. He tells you something that sounds like "It's a good thing there was food on this Starship cuz I worked up a big mighty thirst!"

Hint: The Maw: Killing enemies: Use the following trick to kill the two Hunters, the Sentinels and the Flood. When you are in the maintenance access ways, you will see a black Grunt run from the Flood directly in front of you. Kill the Flood and turn left instead of going straight where the hunters are. Go where the Grunt went. When you get there, the Grunt will be standing in the doorway. Kill him and you should be in an enclosed room with many dead bodies on the ground. When you enter, turn to your right to find be some burning boxes. Go to them, and the Sentinels should be on the other side of the blast door. Shoot the glass and the bullets should go through the door and hit the enemies on the other side. The glass should not break, so the enemies cannot shoot you back.

Hint: The Maw: Destabilize engine: When you open the vents, instead of going to the bottom floor to destroy the engines, jump on the coupling that is moving away from the engine. From there, destroy the engine. When the couplings start to pull back and reach the platform where the computer you pressed to pull the couplings back is located, jump back on it. Note: You cannot make the jump if you are on the white part of the couplings. To make it, there is a brown part on the very end of the couplings. You have to jump from that part to make it across.

Hint: The Pillar of Autumn: Shields: On The Pillar of Autumn level 1, there are some empty airlocks that the Covenants have entered from. After killing the Covenants, go inside to find two extra shields. These shields are better than your normal shield protection.

Hint: The Pillar of Autumn: Different messages from Marine captain: Depending on which difficulty setting you select, the captain of the Marines says a different motivational speech. From something soft in the easy setting, to something brutal with the legendary setting.

Hint: Flashlight: When you get to the part when the people and aliens infected by the flood start coming after you, turn your flashlight on and point it at them. The large one will stop running and jumping so that they are easier to shoot. However, if you raise your gun to reload, turn away from them or take the light off of them in any way, they will resume running around and try to kill you again. Although the creatures infected by the flood stand still when you point the flashlight at them, they may still shoot if they have a gun, so kill them quickly.

.Hint: Do it yourself grenade launcher: Get two plasma grenades. Place a grenade down, then place the second grenade in front of it about one to two feet away. Stand back, and when the first one explodes. it will sent off a duplicate of the remaining grenade. When one of the copies stays, the other blasts off in the direction the first grenade was sent it in. Wait for the second grenade's time to go off and watch the explosion either in the air or on the ground. Note: Do not stand too close, as the grenade may cause injury, burns, and/or even death.

Hint: Rocket Launcher instructions: If you have a gun with a scope, use it to look at the middle of the Rocket Launcher. You will see a small picture with a man holding the gun with the text "Hold This Way".

Hint: Satisfying goals and challenges:

Kill a Hunter with a melee attack (let him rush sidestep, then attack). Kill a Golod Swordman with a melee attack (hit him with a pistol burst from very close, then immediately attack). Run over enemies with a Warhog. Blast groups of enemies with a Scorpion Cannon. Skip a major part of level 5. Shoot down a Banshee with a rocket launcher. Shoot down a Banshee from a Banshee. Finish the game in legendary mode. Playing through the entire game co-op in legendary mode. Taking out a friend (not in a vehicle) in multi-player with a plasma grenade.

Glitch: Crazy camera: In the level 2, "Halo", under the easy difficulty setting, play through the sections normally but make sure that the last lifeboat you find is the one obtained by entering the building to find the marines. When the Pelican arrives for you to get on and fly away, go to your Warthog instead. Drive it up close to the Pelican, making sure that the side seat is facing the opening of the Dropship. If done correctly, the Pelican will fly off, but the camera will not know which vehicle to follow. The camera should fly around and go haywire. Note: This does not happen all the time.

Glitch: Fall to death: In level 5, the section "I Want To Be Your Daddy" is the first that you see. In order to do this trick you have to be in single player mode. When the Dropship is about to let you out, quickly press X button to sit back down in the seat. You will fly down the pit and go around in circles as the Covenant tries to shoot you down. Once you wait long enough, the Dropship disappears from sight and you will float down