The "legendary edition" is sort of a "special-special-edition" that consists of a Full-size replica of the Master Chiefs helmet that contains 4 DVDs with material.
1= The Game
2=Various documentaries and audio commentary
3=Spinoffs such as "Red vs Blue" and "Spark of Life"
4= ALL the cutscenes from all three games in Hi-def
Spiker: the weapon of choice for brutes.
Spartan Laser: An anti-veichle weapon, fires a red laser that pretty much destroys any veichle that is hit.
Moongoose: The ATV that was supposed to be in Halo 2. It carries 2 people and it doesn't have any weapons so it's mainly a means of getting from point A to point B as fast as possible.
There are 3 levels which are revealed,
Valhalla = blood gulch, High Ground = Zanzibar, Snowbound = Frost bite.
There is a new controller setup where you click the right bumper to reload the right weapon and the left bumper for the left weapon.
The X-button will have a special use which Bungie will not reveal.
Note: The images are from an alpha build of Halo 3
Source: Level Magazine