The first-person shooter tower defense Sanctum 2 is free for a limited time through the Humble Store. The game normally costs 12.99 euro but for the next 48 hours you can get it for free.
To get the game, just go to the Sanctum 2 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, November 25 at 10 a.m. PT. But be careful, because the keys cannot be redeemed after December 9.