Xbox LIVE seems to be a hive of activity today, first with a demo for Forza, then XCOM and now the Mass Effect 3: Retaliation multiplayer expansion.
The latest addition to Mass Effect 3 is available for free.
Here's a description:
The Black Arks have arrived! In Mass Effect 3: Retaliation, Harbinger unleashes a new wave of terror on the galaxy: the Collector legions! Battle this all-new fourth enemy faction as well as reinforcements to existing factions like the Cerberus Dragoon and Geth Bomber. Face environmental hazards on the original six game maps, and achieve victory with new ammo powers, gear pieces, kits for the turians and volus, and Collector weapons!
You can add the Retaliation pack to your Xbox LIVE queue here.