I searched to see if this game had been covered here before, but I am decently sure it has not, so I give you a pretty damn awesome 2D puzzle game: Crayon Physics. The idea is really simple, the execution is awesome: using ‘crayons’, you draw boxes to push and guide a ball to the goal on each level (the video uses a tablet PC but you obviously can draw with a mouse). As soon as you complete drawing each box, gravity kicks in and it drops, obeying the rules of a pretty darn cool 2D physics engine.
Crayon Physics Deluxe is en route “when it’s done”, and it will allow you to draw any shape, not just boxes.I guess I should warn you that this game requires a download. But I have seen the old version in action in person, and it is super rad. The above video is the deluxe version, but it shows you the basic concept of how the gameplay works, so if you are intrigued, check it out. The physics engine alone would probably be a great engine to license out to other games makers, hopefully that will happen, because it is certainly a powerful one.