Anno 1602 is free from December 17 to 23

Anno 1602—or 1602 AD, as it was known in North America—was originally released in 1998, which in the context of videogames might as well be 1602. But it's an RTS/management game, which generally hold up to the years better than other genres, and even graphically it's not too bad: Dated but far from ugly. All of which is relevant right now, 20 years down the road, because from now until December 22, it's free for everyone.

To get it, you'll need to log in with your Ubisoft account—you do have a Ubisoft account, right?—at which point it will automatically be added to your Uplay library. You'll be given the option to either launch Uplay or download it if you don't already have it installed, or you can just go off and do something else at this point, confident in the knowledge that if you ever do have the urge to play Anno 1602, it will be waiting for you.