The steampunk airship shooter Guns of Icarus Online is free for limited time through the Humble Store.
The game normally costs 4.99 euro but you can get it for free from the Humble Store until 10 am PT on October 28.
To get the game, just go to the Guns of Icarus Online 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.
But be careful, because the keys cannot be redeemed after November 11.