Phun is a -free for non-commercial use- 2d physics sandbox , it proves that Physics Could be Fun :) with lots of tools that you can use to create great simulations or machines.
According to their main site:
Phun is a free game like 2D physics sandbox where you can play with physics like never before. Children, students, engineers, artists, university professors, and many others, use Phun for telling stories, learning, constructing amazing machines, creating games - or just for the plain fun of it.
for more screenshots and Videos visit phun media page http://www.phunland.com/wiki/Media.
hope you enjoy Physics now :)