Megg

Jaime Griesemer, lead designer on Halo: Combat Evolved, gave his then girlfriend Meg (aka Pallor of Subnova) a gift in the form of an easter egg. The egg, affectionately called the Megg, was hidden in Pillar of Autumn (Level) under a series of bizarre triggers (almost impossible feats) to ensure that no one could find it accidentally. The Megg itself is a heart made from human blood, with an M made from bullet holes in the center.

(NOTE: the 2nd door will still be locked, if you go to it right when the Marines come out. Kill Keyes, stay at that position awaiting the marines to "crowd" one side of the bridge. Than zoom past the "empty" side blazing lead, and into the door way.)
 * 1) Start the level on Legendary, single player.
 * 2) After you gain control, walk toward the canisters behind you (oppisite the exit).
 * 3) Crounch jump on top the yellow canister and then jump back down and move through the level as normal, (skipping the Marine who gets in your way) till you get to the bridge.
 * 4) Once there, run to the other side without triggering the cinematic and jump down into the small glass pit.
 * 5) Wait for the message of how to climb ladders, and then climb up and trigger the cinematic.
 * 6) After you get the Pistol, go through the exit (killing the Grunts) and get the Assault Rifle,then go back to the bridge.
 * 7) Kill Keyes (YOU HAVE TO!).
 * 8) Which will send in the "Invincible Marines" out of a previously locked door, (the right side of the exit).
 * 9) As Cortana finishes saying "The Master Chief has gone Rampant", a 2nd door (right behind the 1st locked door) unlocks.
 * 10) Use the AR to (this is the real trick) keep the Marines at bay, and head in.
 * 1) Enter the hallway, at the end you will find the "Megg" on the ceiling (Don't worry the marines wont come in).

Campaign
There is also a Megg egg located in Halo 2. Discovered by decompiling the game script by "Soldier of Light" from High Impact Halo, Halo 2's version turned out to be much less complicated, while no less impressive. It is activated by going through the first level, Cairo Station, on Legendary, without losing your shields once. Upon reaching the final bomb room, a sound clip will play. The text of the clip is: "Megg, please report to the red courtesy phone. Megg, to the red courtesy phone."

Multiplayer
On Coagulation (Level) there is a Megg that can be viewed in game.

When you finish this go to blue base in Coagulation. Use the Spectre's boost to climb up the right part of the cliff near the base. There is a ridge on the wall as you continue left that forces you down, so stay clear of it by moving slightly down from it.
 * First, change your character model to an Elite, if it isn't already (this is needed because they get out of the Spectre from the front) and make a gametype where you have:
 * an overshield
 * a couple of Spectres.

By now you are completely sideways, and approaching a ridge on the cliff. keep boosting on it, and you'll eventually get over it. It may take a few tries but just be patient and keep trying.

When you get to the very top of the cliff, turn around in your Spectre, keep its left wing on the rim you originally approached, move the nose of the craft slightly over the rim, and search for a small notch in the cliff. I'm not entirely sure whether it was pure luck or timing that got me over it, but you have to get out of the vehicle and hopefully you will be over the level.

Now to find the sign. Walk along the cliff to red base but DON'T walk on the edge! There are notches in it that you can fall into and never get out of. Once at the back end of red base's cliffs, look around for a white patch in the rock. At first it won't look like anything, but back up and look again. There is a "M" clearly visible on the cliff.