DiRT Rally is the most authentic and thrilling rally game ever made, road-tested over 80 million miles by the DiRT community. It perfectly captures that white knuckle feeling of racing on the edge as you hurtle along dangerous roads at breakneck speed, knowing that one crash could irreparably harm your stage time.

Just head over to Humble Store and click on “get the game”. Note that you must be logged in to do this. You do need to subscribe to their newsletter, but that's nothing a quick email provider or an unsubscribe can't fix. 

After you add it to your library you can get the key from your account page or wait for the mail with it. Make sure to activate your key on Steam soon because they’ll go invalid soon after the promo ends. Once the key is redeemed to your account, it’s yours to keep.