The Codemasters racing sim Grid is an older game (it came out in 2008), but a very good one, too. And for the next couple of days, it's also free on the Humble Store. The price, you will notice, is now $0.00, and in case there was any doubt the big green button beside it says plainly, "Get it free!"
Click that, follow the instructions, and in short order you'll be presented with a Steam code. Nip over to Steam, activate it, and faster than you can say, "The Humble Store is also having a Codemasters sale, and god bless us, everyone" you've got Grid in your library.
Two points to be aware of: First, the free offer is only good for the next slightly-less-than-two-days, which by my estimate puts the deadline at 1 pm ET on December 24. And second, the code itself must be redeemed by the end of the month, so when you get it, be sure to use it. Enjoy!