From Major Nelson:

"Celebrate the count down to 2012 with 12 days of deals on games, add-ons, avatars and more from Xbox LIVE.

Drop by Xbox LIVE every day to see what new deals we have for you – but remember these special discounts won’t last more than a day."

Beyond that, there are a ton of deals (check link), including 50% off the excellent (from what I've played) BloodRayne: Betrayal.