Crayon Physics

I just stumbled across this today though it has been available since 2008.  I have been enthralled with the game demo. Basically, the game comes with several levels of "playgrounds".  In the playgrounds, you have an objects (usually a ball) that you need to get to a star object as the goal.  There are objects missing (shapes) in order to complete the goals.  It is up to you to determine what is needed to reach the star.  The fun part of this game is that you use the crayon (mouse or tablet pen) to draw the missing pieces.  Once you release the pen or mouse button, the piece you just drew is dropped into the playground. From here you can either push the ball to start it rolling or if the object you drew was above the ball, when it drops, it will force movement of the ball depending on how the object falls.

There is also a playground for you to submit your own playgrounds as well as download those of other users.  Some of these playgrounds can be quite elaborate and tricky.

Check out the video on the main page for a sample of what this thing does.