While Halo 3 and Assassin's Creed 2 were originally promised, it appears Microsoft has pivoted a bit with today's release. According to Major Nelson, today's free game is Defense Grid: The Awakening. Normally 800 Microsoft Points ($10), the game will be free until July 15th.
You can download the game from Xbox Live Marketplace. As of publishing, the Gold price reduction has yet to go in effect, however.
Fans seem disappointed by this week's offer, especially when the aforementioned games have yet to surface. OXM notes that Microsoft clearly promised AC2 and Halo 3 as free games this month. In response, Major Nelson tweeted: "I don't know what titles will be released when, but I'll let you know what they are when I find out."