Grid, a 2008 racing game by Codemasters, is totally free on PC through the Humble Store. You can get it for free until 1PM ET on Saturday. That means tomorrow, so be quick. But be careful, because the keys cannot be redeemed after 10AM PT on December 31.
You can grab the game here.
To get it, you need to login to your Humble Store account and then click on "Get It Free!". After that, go to your "Cart" and follow on-screen instructions. Once you get your key, you can activate it through the Steam Client. Once you add it to your Steam Library, it's yours to keep forever!