Sony and Microsoft have officially announced the titles that will be released on PS Plus and Xbox Games With Gold, respectively, for the month of December. In addition, in celebration of the holiday season, Microsoft has announced that subscribers will receive an extra game, meaning there will be five free games in December (Sony, of course, continues with their usual six games, two for each platform). And remember, as of November 2015, all games released under the Games With Gold banner will be playable on the X-Box One through backwards compatibility.
Sony
Kings Quest: A Knight To Remember Chapter One (PS4)
Gauntlet: Slayer Edition (PS4)
SSX (PS3)
Far Cry 3: Blood Dragon (PS3)
Freedom Wars (Vita)
Rocketbirds: Hardboiled Chicken (Vita)
Microsoft
The Incredible Adventures of Van Helsing (XBONE)
Thief (XBONE)
CastleStorm (XB360)
Sacred 3 (XB360)
Operation Flashpoint: Dragon Rising (XB360)