The Codemasters racing game Grid 2 is an oldie but a goodie, earning 82/100 in our 2013 review. "More fun than a sim," we said, and "spectacular to look at," with challenging AI, plenty to do, and smooth performance on a mid-range rig. It's a testament to the game's durability that six years after release, it's still selling for $30 at regular price. For the next three days on the Humble Store, however, it is free (opens in new tab).
And not just Grid 2, but also the Bathurst and Spa-Francorchamps track packs, each with new tracks to tear up and new cars to do it with. Just click the "Get the Game" button and follow the subsequent instructions, and Steam keys for all will be delivered unto you. Take note that the keys themselves are time-limited, and must be redeemed on Steam by 10 am PT on March 21, or they will expire and you will be left with nothing.
The Humble Store Grid 2 freebie will be available until 10 am PT on March 17, or while supplies last. Since that does actually happen—a limited number of codes are available for each giveaway—you probably shouldn't dawdle.