We've not discussed Race The Sun too much in the past few years, only to note the troubles it was having without Steam upon release in 2013. I'll remedy that: I love it. It's uncomplicated and beautiful, has just enough skill and strategy to it to be challenging and enough variation in modes to match your mood. It's even got a level creator for you to sculpt your own runs. For the five bucks I originally picked it up at, it was a steal. At the absolute zero you can get it for today on Steam, it's some sort of negative overflow value proposition.
You'll get to keep the copy you reserve today forever, rather than it just being playable for this 24 hour period. Once that time is up it will return to its usual price however, so get in quick. It's being done in celebration of the game launching on iOS devices, which if you happen to own and prefer you can get here.
It's also had some DLC released recently, Sunrise at a poltry £1.59. It removes the time limit of a setting sun and lets you drift forever through a specially designed world. I play it for the challenge of climbing the daily leaderboards, but if something more zen is your style, this sounds like it's for you.