Here's the latest, assembled by too many to thank at bungie forums. Compiled by FrostyWolf.
Everything I know is posted here, so please don't PM me with questions. I'm posting this as we learn.
-Ahoatam
1) In the large room after the armory, you need to use a duck-jump to get onto a light fixture on the right wall (right of the room when u enter) and then jump from there onto the higher area. There is a series of diagonal beams. Walk up one near to the end of it and use it to duck-jump onto one of the rails. You can fall between the rails, so be careful. Once on the rail, walk down it until you can jump onto the highest platform. Then bash the trashcan.
Difficulty: Legendary
Level: Cairo Station
Word: thunderstorm
Effect:


??
Status: Confirmed
2) At the beginning of Outskirts jump on top of the light on top of the door with a crouch jump, then jump to the left platform. Turn to your left and go down the long dark corridor and pick up the skull; it will say blind and your screen will flash black for a while. In this mode you can not see your weapon, body, shields, ammo, or radar.
Difficulty: Any
Level: Outskirts
Word: Blind
Effect: No HUD or weapons on screen
Status: Confirmed
3) Right after u get out of the sewers(Tunnel), there is a big area and 2 ghosts, kill every thing first, then go to the middle of it where u see a arc that leads up onto a building, throw a nade down and jump by it to get enough height to start climbing it. when you get to the top take a left and hug the wall around the structure and the skull will be there, it says "catch"
Difficulty: Legendary
Level: Metropolis
Word: Catch
Effect:


Status: Confirmed
4) I can add my confirmation to the list of those that have found the grunt birthday skull. Here are the instructions on finding it. First it can only be found on Legendary, my guess is that those of you who have been having trouble finding it, you are merely on the wrong difficulty setting, not looking in the wrong place. As for where its at, in the Arbiter level, the very first time you get a banshee, fly down, below you is a large beamlike structure (not to be confused with the one directly below you that turns and goes vertical) running from the arm of the station you just came out of to one exactly like it on the side opposite. Below this structure are 3 cylinders hanging down, with cylinders of light shooting out of the bottom that look like they are what keeps the station floating. If my explanation is too vague, they are the absolute lowest parts of the station. Now go to the middle engine and land on the top part of it. This and the other engines connect to the large structure mentioned earlier. The tops of the engines come up to meet the structure at an angle, where this angle is under the connecting structure and coming up to meet it, there is a place to walk (like being under a bridge). Under one side you will see the six grunts with the skull in the middle.
Difficulty: Legendary
Level: The Arbiter
Word: Grunt Birthday Party
Effect: When you have gotten the grunt b-day party skull, bad guys will explode only when killed by a head shot. Even after their dead, if you shoot their head, they will explode. this is awesome....just shoot a grunt in the face and the dudes around him will die or get damaged.
Status: Confirmed
5) After getting off the long elevator ride, you will come to a room where convent and flood are fighting below you, and you have to break the glass to get to where they are fighting. If you crouch/grenade jump up on the opposite side of where you came down, the skull should be up there.
Difficulty: Legendary
Level: Oracle
Word:

??
Effect:

??
Status: Not Confirmed
6) I found another one in the ruins on Delta Halo. Current mission op: Push through Covenant-held ruins. When you first come out of the big building and see the turret and 5/6 shield up on the wall there is a wall over to the left hand side with the two grunt turrets on it, the big block and door are at the far end of that wall... In the room that the block is *ahem* blocking there are two dead grunts and two plasma swords... There's a door being blocked by a rock. Jump on the rock. Then jump on the ledge that's located above the blocked doorway. Turn left and jump up on another ledge. Grenade jump up to where you'll find Two Red Elites - That become cloaked, surrounding a skull. Pick it up, and it reads Envy. For the ENVY skull, you must play it on Legendary, it makes it so the when you press the white button, you cloak. This cloaking only lasts a few seconds (just like the Arbiter's armor) and can be used on any level and any difficulty after you get it.
Difficulty: Legendary
Level: Delta Halo
Word: Envy
Effect: Cloak instead of Flashlight, 5 second cloak with 10 second recharge
Status: Confirmed
7) Ok on Regret, just after you get off the gondola, and go in the structure, there is a big block of stone, jump on that an you can get on a ledge that leads out to the covie turrets, facing back towards the structure, jump up another level around the right side of the structure, It is pretty straight lined from here. You should find the Skull on a ledge when you make it around. I just got the Skull in Regret, this time in legendary mode. Instead of the "Blind" mode, I got "Assassins". No visible change to the HUD, BUT all enemies become invisible. ALL ENEMIES, including drones and Jackals.
Difficulty: Legendary
Level: Regret
Word: Assassins
Effect: All enemies in game are permanently cloaked, AI cannot see them and so are no help at all.
Status: Confirmed

From the start of the level, look to your left for an elite. To the left of the elite there is a tunnel. Head into the tunnel and go straight until you reach a cliff. Now turn left and follow the cliff till you reach a rock wall. To the right of the wall and the cliff is a narrow passage, skull is at the end of that passage.
Difficulty: Legendary
Level: Quarantine Zone
Word: Sputnik
Effect:
Status: Confirmed
9) Get the tank and there are holes in the level along the walls and take a tank to a certin one (not sure if it will work with others) but the one i used had a cliff right above it and I backed the tank against it and up the hill backwards and got out and got on it and jumped up then had to grenade jump to the next part and from there u can go anywhere and its on the opposite side of the level
Difficulty: Legendary
Level: Quarantine Zone
Word: Infinite
Effect: Infinite energy sword and leap kill from far away
Status: not confirmed - allready getting negtaive feedback on this. Plus the directions are horrible, and one has allready been found on this level, possibly fake.
10) So far, this only works on Legendary. Fight all the way through the Covenant city until you reach the outdoors. As soon as you are outside, stick to the right-hand side and walk along the wall. You should see a Covenant wall thingy with a red rail on it. Jump up to it (crouch jump helps) and at the end there is the Anger skull in a small grav-lift.
Difficulty: Legendary
Level: Gravemind
Word: Anger
Effect:


?
Status: Confirmed
11) On the final grav-lift on "high Charity", look up. A skull will pass you by. Hold X as it passes you to receive the status (the Skull isn't there on Easy, but Legendary).
Difficulty: Legendary
Level: High Charity
Word: Iron
Effect:

??
Status: not confirmed
12) Anyways when you get a banshee and defend the scarab to the place that he blows up the door. But fly above it and onto a railing, don't go too far on it or it will send the banshee flying, now jump out. You should be on the railing which if you look up is aiming towards the middle of the big facility. now go down a rail and head onto the spherical structure part and head to the pillar opposite of the one you were on, you can just walk on up and it is there.
Difficulty: Legendary
Level: Great Journey
Word: Black Eye
Effect: Shield wont regen, when you melee someone you get energy power back, so if you melee someone you get a little bit of shield. So you can get a bit at a time all the way to overshield!
Status: not confirmed