This past weekend, Microsoft made the announcement that Zheroes for Xbox One and Deus Ex: Human Revolution for the Xbox 360 is now free for download. The former will be available between January 16, 2016 and February 15, 2016. The latter, on the other hand, is available only from January 16 until the 31st.
These games have since replaced Thief for Xbox One and Dirt Showdown for the Xbox 360. Both games were also free to download up until the middle of January. In related news, Killer Instinct: Season One Ultra Edition will be available for free downloads until the end of the month for Xbox One.