From December 1-15, subscribers can pick up Gears of War (normally $20) for free. In Germany and Japan, players will receive Halo Wars instead.
Then starting December 16 and running through the end of the month, subscribers can download Shoot Many Robots (normally $10) for free.
The free games are part of Microsoft's Games With Gold promotion, where subscribers can download two free games every month to keep. The program was recently extended to Xbox One.