Stardock's 4X turn-based strategy Galactic Civilizations II: Ultimate Edition is free for a limited time through the Humble Store. The game normally costs 19.99 euro but for the next 48 hours you can get it for free.
To get the game, just go to the Galactic Civilizations II: Ultimate Edition page on the Humble Store, and click "Add to cart" then "Checkout". After that click "Get it for free." Once you get the game, it will be yours forever. The offer ends on Saturday, May 19 at 10 a.m. PT. But be careful, because the keys cannot be redeemed after June 2.