Back in June Microsoft announced the Games with Gold program for Xbox Live Gold Subscribers which would reward subscribers with two free game downloads per month all the way through December 2013.
The first game offered, Defense Grid, will not be accessible after today so if you have not grabbed it then you should do that first.
The next game being offered under the Games with Gold deal is Assassins Creed II and it will be available starting today and is in fact already marked as free for Xbox Live Gold subscribers in the Xbox Marketplace.
The Games with Gold deal will ultimately offer a total of 12 free games to Xbox Live Gold subscribers.