announced at E3, with the first, Defense Grid: The Awakening, still available for free until later today.
Beyond the free stab-o Italiano, there's also some premium Assassin's Creed content on sale until the end of July. If you end up enjoying Assassin's Creed 2, the other two installments in the Ezio trilogy, Brotherhood (great!) and Revelations (oy, with the tower defense), are $15 apiece.
There's also discounts on the downloadable content packs and the season pass for Assassin's Creed 3. Find pricing details on Major Nelson's site or through the Xbox Marketplace starting tomorrow.