GameStooge enjoyed Microsoft's live demo of Halo 3: ODST.
Excerpt: "ODST features a new co-op multiplayer mode called "Firefight", in which four players hold off waves of Convenant attacks. The waves of Covenant are never the same, similar to Left 4 Dead's "Director", so every encounter is different. Each wave has a different theme, such as a wave of Brutes or a wave of Grunts. The "Grand Finale" features three to four Chieftains. Firefights have all of the features of normal competitive play as well - killtaculars, medals, and so forth. Players can activate all-new Skulls that are found in the single player campaign to affect gameplay; the "Tough Luck" Skull, for instance, decreases enemy accuracy. The game also includes a second disc with every single one of the Halo 3 multiplayer maps, so you don't need to pop in Halo 3 to engage in regular multiplayer."