Phun - A fun 2D physics sandbox by Emil Ernerfeldt - seems to be an awesome program that can be a black hole for planned working hours. You can draw physical objects and switch on gravity and friction etc. Have a look at the video. It reminds me on the video MIT sketching which showed a more natural interface (draw your stuff on a board and make it run) but is less powerful. Phun is not (yet) open source but according to the FAQ the author is planning to make the source code available at some point.
[via Bad Science]