Ubisoft is offering the acclaimed RTS World in Conflict now through December 11th. They will then release Assassin's Creed: Black Flag for free from December 12 through the 18th.
https://www.notebookcheck.net/Get-World-in-Conflict-and-Assassin-s-Creed-Black-Flag-for-free-this-month.268054.0.html