I've just updated my website with a physics 'sandbox' I made using one of my favorite algorithms: springs...

Springs let you do amazing things with relatively very simple algorithms (such as simulating mass, inertia and a center of gravity)

http://www.sulaco.co.za/nitrogen/projectinfo.asp?id=29

It lets you create vertexes and link them together to form boxes or ropes (in 2D) then you press play and you can drag the vertexes around under gravity...
If you drag too hard your creation will break under the impact.

It's pretty stable but still not time independent. Something about the way I set out the code makes it stubbornly resist a time-varying physics 'tick'.