The following content is now available on the Xbox Live Marketplace. Update: The latest X06 additions are bolded.
Demos: FEAR, Lego Star Wars II: The Original Trilogy, Sonic the Hedgehog.
Arcade: Doom.
Trailers: Call Of Duty 3, Banjo Kazooie, Marvel Ultimate Online, Viva Pinata, Assassin’s Creed, Splinter Cell: Double Agent, Blue Dragon, Lost Odyssey, Bioshock, 
FIFA 07, Rainbow Six Vegas, Halo Wars, Project Gotham Racing 4, Virtua Tennis 3, NBA Live 07, NHL 2K7, Battlestations Midway, Eragon, Import Tuner Challenge, Need for Speed Carbon, The Darkness, Forza 2 Physics & Damage, NBA 2K7, Sonic The Hedgehog, Shrek 3, Just Cause, Kane & Lynch: Dead Men.
Videos: TGS Showfloor video 2, TGS Showfloor video 3.
Picture Packs: Halo Wars Picture Pack, Halo Wars Theme, Mass Effect Picture Pack, Battlestar Galactica (US Only).
Via Majornelson