Halo 3

Successfully complete the final mission on the Legendary difficulty setting to view an alternate ending.

Complete the indicated task in Campaign mode to unlock the corresponding armor part in Multi-player mode.

Elite Ascetic Body
Earn the "Up Close and Personal" achievement.

Elite Ascetic Head
Earn the "Steppin' Razor" achievement.

Elite Ascetic Shoulders
Earn the "Overkill" achievement.

Elite Commando Body
Earn the "Triple Kill" achievement.

Elite Commando Head
Earn the "Overkill" achievement.

Elite Commando Shoulders
Earn the "Killing Frenzy" achievement.

Elite Flight Body
Complete "Tsavo Highway" under the Heroic or Legendary difficulty.

Elite Flight Head
Complete Campaign mode under the Heroic difficulty.

Elite Flight Shoulders
Complete "The Ark" under the Heroic or higher difficulty.

Spartan EOD Body
Complete "Tsavo Highway" under the Legendary difficulty.

Spartan EOD Head
Complete Campaign mode under the Legendary difficulty.

Spartan EOD Shoulders
Complete "The Ark" under the Legendary difficulty.

Spartan EVA Body
Complete "Tsavo Highway" under the Normal or higher difficulty.

Spartan EVA Head
Complete Campaign mode under the Normal or higher difficulty.

Spartan EVA Shoulders
Complete "The Ark" under the Normal or higher difficulty.

Spartan Mark V Head
Earn the "UNSC Spartan" achievement.

Spartan ODST Head
Earn the "Spartan Graduate" achievement.

Spartan Rogue Head
Earn the "Spartan Officer" achievement.

Spartan Scout Body
Earn the "Too Close to the Sun" achievement.

Spartan Scout Head
Earn the "Used Car Salesman" achievement.

Spartan Scout Shoulders
Earn the "We're in for Some Chop" achievement.

Spartan Security Head
Earn 1,000 Gamerscore points.

Spartan Security Shoulders
Earn 250 Gamerscore points.

Score over points in the Campaign meta-game on the fourth mission.

Finish the fourth mission of the Campaign on Normal, Heroic, or Legendary.

Load up Assembly in forge mode and go into monitor mode (press up on the d-pad). There are two "arms" attatched to the centre building. In monitor mode, fly up and into the small opening at the top of the arm (it's one of the two).