From Major Nelson:
“For the month of January, Xbox Live Gold members will receive four new free games – two on Xbox One and two on Xbox 360 – as part of the Games with Gold program. You can play both Xbox 360 titles on your Xbox One with Backward Compatibility.
On Xbox One, Xbox Live Gold members can download Killer Instinct: Season 1 Ultra Edition($39.99 ERP) for free during the month of January. Zheros will be available as a free download from January 16th to February 15th.
On Xbox 360, starting Friday, 01 January, DiRT Showdown ($14.99 ERP) will be free for Xbox Live Gold members through January 15th. Then on January 16th, Xbox Live Gold Members can download Deus Ex: Human Revolution ($14.99 ERP) for free through January 31st.”