Kinect: Sports.
I was surprised at how addicting the little game was, yes it wasn't very hard to play, but it was very fun, and it also got me a lot of easy achievements.
The game is by no means perfect but it tries, there are in game advertisement every once in a while, and the controls can be sloppy. Of course this is the Kinects first year on the market and as the years pass they will probably polish or update the Kinect so that its more realistic and interactive. There have also been arguments between the Wii and Kinect about which is more innovative and fun to play with. I personally think the Kinect beats the Wii (nothing against the wii) but I think its more fun. Also I couldn't see first person shooters using this mechanic in years probably due to the fact that I prefer a controller. Overall if you have the Kinect go rent this game.