Today Microsoft announced a limited-time deal they're about to run for the Xbox One in the States. If you a buy an Xbox One (including one in a bundle) between the 7th and 13th of September you'll be able to choose a free game of your choice.
Although Call of Duty: Ghosts, Watch Dogs, Diablo III: Reaper of Souls, Titanfall, Plants vs. Zombies: Garden Warfare, and Madden NFL 15 are the games featured in promotional material for the deal (see image above), you'll actually be able to select any Xbox One retail game, the only restriction being that you cannot pick a game that has yet to be released (i.e. pre-order games).
For full terms and conditions head on over to the Xbox Wire blog post.