Have you played with Crayon Physics yet?

Crayon Physics is the latest experimental project by Petri Purho
"You play with crayons and physics. The goal of the game is to move the red ball so that it collects the stars. You can cause the red ball to move by drawing physical objects"
This game is still in development, you can download from- Kloonigames - Crayon Physics Deluxe
Unfortunately there's no Mac version:(