Race the Sun is a procedurally-generated endless runner in which you take control of a solar-powered aircraft and fly it through increasingly difficult obstacle courses at what feels like increasingly ludicrous speeds, all while trying to keep pace with a slowly setting sun. It's simple but really quite good, in a 'shouting surprised obscenities as you smash into a wall' kind of way, and today—and only today—it's free.
The freebie marks the release of Race the Sun on the App Store (opens in new tab), although Aaron San Filippo, the co-founder of developer Flippfly, described it as a "crazy experiment" on Twitter. It also follows closely on the release of the Sunrise DLC, a "'zen' type mode" of play that eliminates the setting sun, increasing difficulty levels, and leaderboards. At the same time, the game was updated to add a first-person camera view and "High Scores in the Sky," which lets you literally fly past your friends' high scores.
Race the Sun is free now and will remain so until 1 pm PDT on July 31, which is tomorrow. Grab it from Steam.