"Legendary Edition" - box is a replica of Master Chief's helmet, includes 4-disc set:
  • Disc one: game
  • Disc two: documentaries and developer commentary
  • Disc three: spin-off material (Red vs. Blue, Spark of Life, etc.)
  • Disc four: all cut scenes from Halo 1-3 in HD
  • Bonus: sneak peek of Halo movie will be included on one of the discs (maybe)
Sorry for reviving this semi-old thread but I have a reason. For those who don't know, Gamestop has started taking pre-orders for the set above. The Price: $99.99, and to pre-order you would need to put $25 down on it. Though when I asked one of the guys working what exactly was included, he of course had no idea.

Edit: Here's a link