Phun is a Master of Science Thesis by Computer Science student, Emil Ernerfeldt at the VRLab, Umeå University. It is a 2D testing ground where the user can draw and insert objects into the game and apply physical attributes them and then watch them interact with each other. It is meant to be both entertaining and educational.
Phun is meant to be a playground where people can be creative. It can also be used as an educational tool to learn about physics concepts such as restitution and friction.
It is currently in Beta 3 and available for download here. In my physics playground, I made some wheel-boobs that rotate, bounce, and finally float around in the water after being run over by my giant bubble tank. I have a lot of time on my hands.