ok these are all i can think of, and there are a lot.....
cuz i spend half my life on youtube.
Hearts on sierra 117: hidden all throughout the first level of the game, you can find hearts with the names of some of the creators, their girlfriend/wife, and dates.
Talking grunt: there is a grunt to the left of the ramp that leads into the hangar on the level "Halo". just before you go off the ramp, get out of the warthog, look left, and you should see him out on a ledge. this is what he says:
"Hey demon! The jerk store called and they're all out of you! Poor you, stolen at the age of six and conscripted into the military, nyuuugh! Okay look, you let me live, I got the Fist of Rukt. I'll be the bottom. I'll polish your boots. I'll polish your helmet! It's the gas, haha! When I'm on the gas, I don't know what I'm doing half the time!" hehehehehehehe.
sandtrap easter eggs: they are on top of the pillars out in the desert area in sandtrap. you can see them by:
overload the boundaries with trip mines all the way out to the pillar. go into theater mode, ff to the part where you did this, and just fly up and look at them. they are actual eggs.
Face on the moonn the map standoff, if you look at the moon, you can see a face in it. if you look closely, it is actually the face of a bungie employee. He's bald.
Half jawwed elite in crashed floodship: on the level floodgate, when you jump down in the hole to enter the floodship, there's a few dead ultra elites. sometimes the last one is actually Rtas Vadum, the elite from halo 2 with half of one jaw missing.
Last resort cloud: on the level last resort, if u go 2 the beach and look around in the sky, the brightest cloud looks like a spartan flexing his arm muscles.
The half naked guy: on the last level, (halo) near the seventh marathon terminal, besides having the terminal's room, there are two other rooms. one has a vertical energy beam, and the other, a horizontal and a 1000 ft drop. fun anyway, on the left side of the chasam there is a little ledge. on your replay, you can move the camera in to the room and look at the guy. he's half naked.this is the man on the missing poster. (wait, it will come)
High ground music: on xbl you can go into the base. it has a room with two big green thingies. if u look with sniper scope, you see that it is a radio. just wait inside the room for a while, and it will start playing a randm song from one of the halos. 1, 2, or 3.
Morse code: the same radio as mentioned above, but sometimes it plays morse code. the code reads:
"Frog blast the vent core"
Missing person poster: on the level floodgate, there is a poster that depicts a missing person. that person is the same person as in the half naked guy, and is one of the creators.
after the flood cross the rooftops and turn left,(where the buildings start) past the brutes bodies, and the grav hammer it will be lying on the floor and you can read it by zooming.
Boat that looks like master chief's helmet: on the level, the storm, there is a boat in the first open area of the level. (besides the one you drive in on) to the right of where you come in, there is a big crashed boat. it looks like masterchief's helmet.
Whizzing brute: level, the ark, in the room with the sleeping grunts (after you kill the scarab) there is a urinating brute in the far corner, and he has no idea you're spying on him.
that's about it.........