Grand strategy game Crusader Kings 2 is now free on Steam but we don't know for sure when the offer ends. Probably, you can grab it for free until the end of PDXCON on October 20. The game normally costs 39.99 euro.
To get the game, just go to the Crusader Kings 2 page on Steam, and click the "Free To Play" button. Once you get the game, it will be yours forever.