Awarding winning 2D physics based puzzle/sandbox game game Crayon Physics Deluxe is free for 24 hours.
In the game, you get to experience what it would be like if your drawings would be magically transformed into real physical objects. You have to solve puzzles with your artistic vision and creative use of physics.
The game was part of two Humble Bundles. Check out a gameplay video:
Just visit this page to grab your free copy.