Heads up that Crusader Kings II is "free to play" on Steam right now:
Not sure if it's Free-to-play forever, or just for this weekend to coincide with their PDXCON event - so...
--- OFFER HAS ENDED ---
Humble Bundle is currently giving away the turn-based fantasy strategy game "Age of Wonders III" for free via Steam keys for Windows, Mac and Linux.
Offered while supplies last or until 10am Pacific time, May 11th 2019. (A little under 23hrs at time of posting)