NanaOn-Sha, developer of the amazing PaRappa The Rapper, came out with a Kinect game earlier this year called Haunt. It didn't really make a splash, and odds are you probably haven't heard of it. That all changes today, as you Americans can get the game for free right at this very moment, up until July 2nd.
All you have to do is load up your 360, access the "Kinect Central" button on the main menu, and download the game. To be clear, you can't do it online at Xbox.com.
Some Joystiq users are reporting that you need to have Kinect connected (ha!) in order to see this offer. Others are saying that it comes up listed as 800 MSP rather than free, so double check before you mindlessly mash the purchase button.
I can confirm that I was able to download it fairly easily, and free, just moments ago -- check out my gallery picture for a heads-up on the location of the offer.