I just spent an embarrassing amount of time playing a freeware game called Crayon Physics. You draw rectangles with your mouse, and when you finish drawing gravity takes over and they drop. The goal is to use the rectangles you draw to move a ball towards a star. It sounds kind of stupid, but I swear, once you start it's hard to stop!
After I got through all the levels, I kept playing, drawing levers and seesaws and making rectangles go flying. I'll probably dream about it tonight. Download it here:
The game's developer is working on Crayon Physics Deluxe, where you can draw wheels and axles in addition to rectangles. I signed up to be notified when the game goes live.