In much the same vein as the Powder Game, Incredibots is an in-browser physics simulation sandbox. Instead of simulating the properties of different sorts of matter, it focuses on the physics of simple machines and allows one to create surprisingly complex contraptions (given time and effort- just load up a few "bots" on showcase from the enthusiastic community).
There are challenges to be completed and a tutorial available (which is recommended- some of the required controls and techniques are more intuitive than others). I had the most fun just dinking around in the sandbox mode and seeing what I could come up with (almost a working 4-legged robot... maybe next time).
Try(?)