I wanted to recommend NightSky for a little while now.
A must-have if you enjoy puzzle games based on physics!
You control a mysterious black ball going on an adventure through several canvas-looking shadowed landscapes; such as Industrial, littorals, forests, caves, ruins, etc; resolving many puzzles by using the physics and your two main powers (Speed-up/Slow-Down).
Now that should sound fairly simple, but you'll quickly notice how mean some of the latest levels really are.
The lenght is actually very good! There's a dozen of chapters, each of them containing ~30 puzzles with very interesting mechanics, such as reversed gravity, momentum control, improvised pinball, balancing structures, different vehicles or even giant Rube Goldberg machines. And a very nice work on the variety of their layouts!
When you're done with the game, you have the opportunity to start the "Advanced mode".
That name simply hides a great challenge, for every single level has been modified and revisited to be harder and very, VERY challenging!
As for the explorers and other tricky challenge hunters, there's a few stars hidden in some levels, both in the normal and advanced mode. Brace yourself if you want to retrive them all!
Beware, skills required,
You won't have time to get bored!