The game cannot start because all players are set to observer. Too many players for splitscreen. Only 2 players may play splitscreen coop on the same Xbox 360 console. The party size is too large to start the game. Up to 4 players may play coop on Xbox Live or System Link. Loading information from Xbox Live…

Weapons

Assault Rifle
Battle Rifle
Beam Rifle
Brute Shot
Carbine
Excavator
Gravity Hammer
Flamethrower
Magnum
Missile Launcher
Needler
Plasma Pistol
Plasma Rifle
Fuel Rod Cannon
Rocket Launcher
Shotgun
SMG
Sniper Rifle
Spartan Laser
Spiker
Sword

Vehicles

Mongoose
Warthog
Hornet
Pelican
Scorpion
Ghost
Brute Chopper
Spectre
Shadow
Banshee
Phantom
Wraith

Game Types

Capture the Flag (Invade your opponent’s stronghold, heist their flag, and bring it back to your base to score.)
Slayer (Kill as many of your opponents as you can. The player with the most points wins.)
Oddball (Find the ball and hold on to it to earn time. The player with the most time wins.)
King of the (Hill Gain control of the hill to earn time. Keep your opponents off and earn the most time to win.)
Juggernaut (Work together to take down the Juggernaut. Beware, you kill the Juggernaut, you become the Juggernaut, and only the Juggernaut can win.)
Territories (Find and control the territories on the map. The more territories you control, the faster you earn time.)
Infection (Destroy the Flood or become infected. Score points for killing flood or big points for infecting a human. The player with the most points wins.)
Assault
Da bomb!
V.I.P. (Protect the V.I.P. from dying at the hands of your opponents. If you are the V.I.P., you must stay alive or your opponents score a point.)

Variations (Apparently, you'll be able to share these with other users much in the same way you do with movies )

Team Slayer
Team Swords
Team Rockets
Team Shotguns
Team Fiesta
Team Phantoms
Slayer Duel
Sword Duel
Rifle Duel
Rumble Slayer
Rumble Swords
Rumble Rockets
Rumble Shotguns
Oddball
Fiesta Ball
Team Ball King
Phantom King
Crazy King
Team Crazy King
Multi Flag CTF
CTF Wraiths
CTF Classic
Shotgun CTF
1 Flag CTF
1 Flag CTF Fast
Multi Bomb Assault
Rapid Assault
Neutral Bomb
Neut. Bomb Fast
3 Plots Land Grab

A glitch in the beta allowed users to access the custom games menu option, revealing the following:

Damage resistance
Shield multiplier
Shield recharge rate
Immune to headshots
Grenade counts
Infinite ammo
Weapon pickup disabled
Player speed (25%, 50%, 75%, 90%, 100%, 110%, 125%, 150%, 200%, 300%)
Player gravity (50%, 100%, 200%)
Vehicle use (passenger only)
Motion tracker range (10m, 25m, 75m, 100m, 150m)
Appearance
Vis effects (black glow, team glow, white glow)
Active camo (good camo that might fool and elite, poor camo that might fool a grunt, invisible)
Indestructible vehicles (on, off)

Features :

• Next-gen advantage: Taking full advantage of the power of Xbox 360, Halo 3 expands on everything that has made the Halo franchise great, adding a wealth of technical and gameplay advancements.
• Evolution of design: Next-generation high-definition visuals, enhanced A.I., an advanced real-time lighting engine, and new weapons, characters, and challenges are just a few examples of Halo 3's evolution of design and technology.
• Unparalleled experience: Ultimately, these improvements provide gamers with an unparalleled experience and, in the end, tell an even more compelling and engrossing story.
• Online multiplayer: Working in concert with Xbox Live on Xbox 360, Halo 3 builds upon the unique social multiplayer experience and innovative, evolving online gameplay of Halo 2.

Halo 3 will be available to audiences around the world starting Sept. 25 and will release in Europe on Sept. 26.