Bungie Studios is known for not giving any cheats in their games, however they do give pretty cool Easter Eggs that only the most experienced or bored players will find. Halo is no exception to this and there are several eggs I will be going through. There is also a comprehensive list with screenshots on halo.bungie.org
Before the game even begins there are several Easter eggs in the game menus, starting with the profile creation screen. If you enter the name .fortune into the profile creation menu you will be given a fortune. There are ten in all and this egg stems from one of Bungie's earlier games, Myth. This .fortune program actually was born in the 1970's when UNIX was just being formed. A version of the program was included in Myth's multiplayer on bungie.net. If you want to see the fortunes but don't want to do it yourself there are screenshots from HBO.
Also a few great Easter eggs are on the Multiplayer: Edit Gametypes Menu. Here is a picture of the MOLJNIR armor that the Master Chief wears. Stemming off of this picture are pointers with text that is not easily read. Turn up the resolution and try to make them out by yourself or see HBO's screenshot that has most of it complete here. My favorite is “All your base are belong to us.”
There are a few eggs that are in a few or all levels because they are on the models of objects. One of these is on the Master Chief's boot. If you take out a sniper rifle in co-op and look at the bottom of the chiefs boot you will see the letters MRL. These letters are the initials of Marcus R. Lehto, Halo's art director.
Another egg that appears in a few places is on the Scorpion Tank. You can see this egg from any mode and in any level where the Tank is present. Simple go to the front and look at the set of six numbers on the left front tread, 030569. This is the birthday of Halo's art director, Marcus Lehto.
An egg that had one of the largest and most fun searches of all time was the hunt for the Hippo of Halo. This is a fun one to hunt for yourself if you have the time and a quality TV. This forum post from HBO gives you the clues you need to start your search. For those of you that are lazy I will tell you where you can find the famous Hippo of Halo. It is the most important part of one of the deadliest weapons of Halo, the shotgun. If your TV is good enough take a peek at the shells of the shotgun to see a big smiling hippo face.
Another weapon has a few Chinese symbols on it, but once again you are going to have to be able to get a real close view. I would suggest playing either co-op or multiplayer and getting the sniper rifle to inspect each gun for the symbols. Once again if you are lazy keep reading and I will tell you. The pistol contains two Chinese symbols, one 7 and a symbol for power. HBO has links to all the forum posts and talk about the origins of these symbols and why they were added in.
Before I get into the level specific Easter Eggs there is one more that appears in a few places. Anytime you encounter Captain Keyes pull out a sniper rifle and take a look at his shirt. Beside the marathon symbol there is also a nametag that reads “HELLOMYNAME” and on the line below it “KEYES.” No one has an explanation as to why it does not read “HELLOMYNAMEIS.” But it is still a good Easter egg to find.