Windosill is an odd interactive toy disguised as a sequence of puzzles for the players to solve, available to play on both Windows and OS X operating systems. The game basically involves searching for a key cleverly hidden somewhere in each scene, and once you've found this particular item you can then use it to unlock the door that hinders your progress from the current room to the next.

Access to the first half of the game is free, but to play the second half you will need to purchase an activation code that costs three US dollars. (source, walkthrough)