Beyond the Chiron Gate is a brilliantly-made Twine game available for $10 on Windows, Mac, and Linux. I've been playing it for days and it's a brilliant story - it's going to take a few tries to get the hang of it, but it's practically addicting, the simple gameplay makes it really easy to tell yourself "just one more planet" ... and then it's 3am and you have work in the morning. If you're a completionist, getting all the achievements (contained within the game itself) will definitely keep you occupied for ages.
Additionally, the world of this game is really well fleshed-out and the way information is revealed is very organic. It's a brilliant piece of storytelling, and the end of the game - well, I won't spoil it, but it's truly incredible.
Also, the music's really good.