Coil

On the surface Coil looks a lot like flow at first. Of course it's just the surface and as soon as you start playing you'll begin to realize that Coil is something much more interesting.

With absolutely no instructions you're goal is to simply follow the story and complete a small batch of mini-games. What's great about this title is that you're left on your own to determine what exactly is required of you. Each level is a bit harder than the last and each level builds slightly on the gameplay of the level immediately before it. The short story pieces that occur in between each level are also interesting despite being minimalistic.

While the game does feel unfinished it's free, and despite being relatively short it's an intensely gratifying experience.

Found on TIGSource