Update: I've been reminded that Fable III was technically the FIRST game in the series of free offerings. So yea, that happened.
Microsoft is kicking off its Games with Gold promotion, which was originally announced at this year's E3 conference. The promotion will make two Xbox 360 games free each month, and the next title up for grabs is Hidden Path's 2009 XBLA tower defense game Defense Grid: The Awakening.
Normally priced at 800 Microsoft Points ($10 real human money), Defense Grid will be available for Gold members to download for free from today through July 15. At that point another free game will take its place.
When Microsoft first made the Games with Gold announcement, the two titles used as an example of the free treats to come were Halo 3 and Assassin's Creed II. We'll keep you posted on when those and other free games become available.