Xbox360Achievements.org has managed to get their hands on the official achievements listing for Halo 3.

There are Forty-nine achievements in all, these range from the standard (complete the game on easy) to the more complex, "we need some more achievements" types like: "Overkill - Kill 4 enemies within 4 seconds of one another in a ranked free for all playlist." Achievements that most of us will never, ever see unless we accidentally unlock them while drunk.

Full List Of Achievements (source Xbox Achievements)

	Landfall	20
	Finish the first mission of the Campaign on Normal, Heroic, or Legendary.
	Holdout	20
	Finish the second mission of the Campaign on Normal, Heroic, or Legendary.
	The Road	20
	Finish the third mission of the Campaign on Normal, Heroic, or Legendary.
	Assault	30
	Finish the fourth mission of the Campaign on Normal, Heroic, or Legendary.
	Cleansing	30
	Finish the fifth mission of the Campaign on Normal, Heroic, or Legendary.
	Refuge	30
	Finish the sixth mission of the Campaign on Normal, Heroic, or Legendary.
	Last Stand	40
	Finish the seventh mission of the Campaign on Normal, Heroic, or Legendary.
	The Key	40
	Finish the eighth mission of the Campaign on Normal, Heroic, or Legendary.
	Return	50
	Finish the final mission of the Campaign on Normal, Heroic, or Legendary.
	Campaign Complete: Normal	125
	Finish the Campaign on Normal.
	Campaign Complete: Heroic	125
	Finish the Campaign on Heroic (unlocks achievement for Normal, if not earned).
	Campaign Complete: Legendary	125
	Finish the Campaign on Legendary (unlocks achievements for Normal and Heroic, if not earned).
	Iron	10
	Finish the Campaign on Normal, Heroic, or Legendary, then find and claim this skull.
	Black Eye	10
	Finish the Campaign on Normal, Heroic, or Legendary, then find and claim this skull.
	Tough Luck	10
	Finish the Campaign on Normal, Heroic, or Legendary, then find and claim this skull.
	Catch	10
	Finish the Campaign on Normal, Heroic, or Legendary, then find and claim this skull.
	Fog	10
	Finish the Campaign on Normal, Heroic, or Legendary, then find and claim this skull.
	Famine	10
	Finish the Campaign on Normal, Heroic, or Legendary, then find and claim this skull.
	Thunderstorm	10
	Finish the Campaign on Normal, Heroic, or Legendary, then find and claim this skull.
	Tilt	10
	Finish the Campaign on Normal, Heroic, or Legendary, then find and claim this skull.
	Mythic	10
	Finish the Campaign on Normal, Heroic, or Legendary, then find and claim this skull.
	Spartan Program Recruit	10
	Win your first game in matchmaking to sign up for the Spartan Program (Online).
	UNSC Spartan	15
	Earn your Sergeant rating to be recognized as a true Spartan (Online).
	Spartan Officer	25
	Advance to the Spartan Officer ranks (Online).
	Two for One	5
	Score a Double Kill with a single spartan laser shot in a ranked free for all playlist (Online).
	Too Close to the Sun	5
	Destroy an enemy banshee with the spartan laser or missile pod in a ranked playlist or in campaign.
	Triple Kill	5
	Kill 3 enemies within 4 seconds of one another in a ranked free for all playlist (Online).
	Overkill	5
	Kill 4 enemies within 4 seconds of one another in a ranked free for all playlist (Online).
	Lee R Wilson Memorial	5
	Score 5 grenade sticks in any ranked free for all playlist (Online).
	We’re in for some Chop	5
	Destroy an enemy vehicle with equipment in a ranked playlist or in campaign.
	Killing Frenzy	5
	Kill 10 enemies without dying in any ranked free for all playlist (Online).
	Steppin’ Razor	5
	Score a Triple Kill with the sword in a ranked free for all playlist (Online).
	Mongoose Mowdown	5
	Splatter an enemy with the mongoose in a ranked free for all playlist (Online).
	Up Close and Personal	5
	Kill 5 enemies by melee or assassination in a ranked free for all playlist (Online).
	Fear the Pink Mist	5
	Kill 5 enemies with the needler in a ranked free for all playlist or in campaign.
	Headshot Honcho	5
	Kill 10 enemies with headshots in a ranked free for all playlist or in campaign.
	Used Car Salesman	5
	Destroy a vehicle that has three enemies in it in a ranked playlist or in campaign.
	Marathon Man	40
	Locate and access all Terminals in the Campaign.
	MVP	5
	Earn the MVP in any ranked playlist (Online).
	Maybe Next Time Buddy	5
	Board the same vehicle within 10 seconds after being boarded in any free for all playlist (Online).
	Demon	10
	Score over <X> points in the Campaign meta-game on the second mission.
	Guerilla	10
	Score over <X> points in the Campaign meta-game on the first mission.
	Cavalier	10
	Score over <X> points in the Campaign meta-game on the third mission.
	Askar	10
	Score over <X> points in the Campaign meta-game on the fourth mission.
	Exterminator	10
	Score over <X> points in the Campaign meta-game on the fifth mission.
	Ranger	10
	Score over <X> points in the Campaign meta-game on the sixth mission.
	Vanguard	10
	Score over <X> points in the Campaign meta-game on the seventh mission.
	Orpheus	10
	Score over <X> points in the Campaign meta-game on eighth mission.
	Reclaimer	10
	Score over <X> points in the Campaign meta-game on the final mission.