Faster Reload Trick
This should work for all guns that can reload except for shotgun.
Reload the gun, as soon as you see the clip into the gun animation, either switch guns or melee. The reload would save you good time.
Works in all setting, including multiplayer

Get New Ending Sequence
Finish the game using legendary difficulty setting in Campaign Mode

Hidden Grunt
Warning! This Easter Egg may require you to start the level again.
On the last level(The Maw) when Echo 419 gets shot down, go down aways until you see a tunnel on your right. Get out of the warthog and go through the tunnel on the right. You should be in a corridor with another corridor to your right. Go there. In this corridor there should be a grunt. He won't shoot you or anything. Go up to him and see what he has to say

How to see longer cinemantics
Depending on the difficulty, more cinema is included in the beginning of the level. For example, on the first level, The Pillar of Autumn, choose difficulty on heroic and more cinema (movies) play in the beginning than if the setting was on easy. Another thing, you only have to beat the Maw on legendary setting to see the longer ending

Hunter Tricks
Its like playing with a bull (a bull with a fuel-rod gun and a bad attitude) with a red flag. Wait for it to melee you and right before it hits you move out of the way and hit it in the back. Then jump out of the way so it doesnt kill you. This trick is very hard to do however, try and get behind one hunter so the other one shoots at you and hits the other one in the side...or head..or wherever it might hit it. i did this once and it killed the hunter

Hunters....The easiest way
When dealing with Hunters...why not conserve ammo? All you need is a pistol with at least 2 shots (depending on how accurate you are). Get close to one, wait for it to charge toward you, and quicly side-step. As soon as it gets past you, fire at the exposed orange skin on his back. It also works on the neck...but you'll rarely be able to get a good neck-shot off. And as far as I know...Only the pistol can take them down in one shot

Infinite Ocean Glitch
On the silent Cartographer level, you can take a Warthog and drive out into the ocean as far as you want with no end

Pelican Ride to Death Glitch
Go on the level Assault on the Control Room. At the very beginning you'll get off the Pelican. Right away tap x and you'll get back on the Pelican. Now you'll fly with the ship and eventually enter a mist and full to your doom and "press x to flip the Pelican" will appear on the screen

Shoot enemy through wall
In multi-player mode, drive any Warthog close to a building so that the side of it is touching. Get in the gunners turret and turn it towards the wall. If done correctly and you are close enough, you should be able to to shoot at an enemy through the wall and not receive damage from them if they shoot back

Tip for Driving Warthogs
When driving a warthog before you drive off a cliff spin right away to the side and the warthog won't tip but will stay horizontal and land from and height,(this won't work if the cliff goes off that certain level). Also when turning to make a perfect turn, only do lots of little movements on the right thumb stick. Do this so that the warthog doesn't skid out from big turns and you can control it better

Tips for Killing Elites in Legendary Dif
Here's a good way of killing elites or hunters in legendary. get a needler and some ammo and just unload it on an elite and watch the show --NOTE-- You have to get in close enough for the needles to find their mark, and the delay until they explode leaves you wide open for their plasma rifle, so bring a backup weapon.

For the hunters, unload an entire clip at their backs so the shot doesn't reflect off their shield and start shooting at them with another weapon while waiting for the needles to explode