Sony declared the following games will be free to PS+ members in December 2018.
Iconoclasts, PS Vita (Cross Buy with PS4)
Papers, Please, PS Vita
All games will be available from December 4, 2018 to January 1, 2019.
The Warframe: PlayStation Plus Booster Pack III is also free and includes 100 Platinum, 100,000 Credits, Quanta (a Corpus laser rifle), Quanta Obsidian Skin, 7-Day Affinity Booster, and a 7-Day Credit Booster. PS Plus members can get the pack for free from now until January 15, 2019.