You may have already seen and played this already, but I came across a fantastic 2d physics sandbox game over here ->
http://www.acc.umu.se/%7Eemilk/index.html
You can draw your own shapes in 2d using the mouse and then give them physical properties, you can also make things like springs, water, motors and all sorts of other things, your mind is the limiting factor :)
Check out the youtube video ->http://www.youtube.com/watch?v=0H5g9VS0ENM