The Mass Effect 3 From Ashes DLC was accidentally, and briefly, sold on Xbox Live Marketplace.
From Ashes contains an extra squad member. Spoiler below.
The DLC is bundled (for free) with the N7 Collector's Edition and PC Digital Deluxe Edition.
The extra squad member is a Prothean - the race referred to as Collectors for most of the second Mass Effect game.
The From Ashes Xbox Live Marketplace page has now vanished.
Those who saw the content before it disappeared said it measured 628.18MB and cost 800 Microsoft Points.
BioWare associate producer Michael Gamble later confirmed on the company's forum that From Ashes will be available to buy at launch on all platforms. Let's hope the Prothean squad member is more deeply integrated than Mass Effect 2's DLC character Zaeed Massani, whose conversations were largely non-interactive.