Why Am I Here
The Why Am I Here easter egg can be found on Beaver Creek on Halo: Combat Evolved. Walk up to where the Sniper Rifle spawns, then head for the end of the path. Once you have made it to the end of the path, jump past the dead end. On the right side, there should be a place that you can jump onto. Then, there are some rocks that you must push down. Where the big one used to be, there should be "Why Am I Here" and some initials. If you still can't see it, move back a bit. Try zooming in to see it.

Bulletin Board
On the first and last of the Halo: Combat Evolved levels, a bulletin board can be found just outside the bridge of the Pillar of Autumn. Some of the messages look plausible, some are just silly, others are Easter eggs.

One says: "Missing: Calico Cat: Answers to Jonesy" This is a reference to Alien and Aliens, two movies that Halo borrows from many times, to include the banter of a Marine soldier. Jonesy was a calico cat in the series, the only survivor of the first movie, along with Ripley.

There is a picture of Earth from the Moon with the caption "Fight for her" on it. This is likely propaganda within the UNSC. It also shows Africa, and may be a reference to where the Ark's portal is hidden

There is a note reading:
For Sale:
Pentium XI 33 GF
Chicago Office
Colony Ship
*3 Speed Manual
*Transmission
Hyde Park Condo
Integrity

Hellomyname Keyes
Hellomyname Keyes is an Easter Egg on the level Truth and Reconciliation in Halo: Combat Evolved, you will need to save your sniper rifle until you save Captain Keyes. Zoom in 10x and if you look closely at the name tag on Capt. Keyes, you will see above it, says HELLOMYNAME then below it KEYES.

Albert Einstein on the Halo:
In Halo: Combat Evolved, when the main menu first comes up, look at the top left corner and wait for 6 and a half seconds, soon, you will see a cartoonish face of Albert Einstein. He is on the back of the Halos, so he drifts across the screen (he passes through the A and the L in the word HALO) and then goes off the bottom right corner of the screen.

Birthdate on Tank
The Birthdate on a Tank is a six digit serial number on the left front tread of the Scorpion Tank in Halo: Combat Evolved. It is 030569. It just so happens that Marcus R. Lehto, Halo's Art Director, was born on March 5, 1969.

Talking Grunts
There are three instances of Talking Grunts that have special dialogue that are found in the form of easter eggs. The first is on the final level of Halo: Combat Evolved. When you are escaping the Installation in the Warthog, you will come to a large jump. If you exit your Warthog and walk to the end of a nearby tunnel, you will see and hear the Thirsty Grunt.