Phun is a typically Swedish invention - functional, free, educational but fun at the same time. If you're studying physics then you should get a lot of enjoyment out of this "game".
The game allows you to explore the forces of gravity and friction plus aspects of density, elasticity, viscosity and inertia in a "virtual sandbox". This basically allows you to create two-dimensional objects and then see how these react according to different physical forces. The game is hugely effective at creating realistic states and effects and forces on your objects. It's a shame that it doesn't support 3D objects although the developers say this has been done to make the interface simpler.
To think that this project was developed by a student at Umea university is quite amazing too. In this latest version, they've ironed out many of the bugs that were plaguing the program. For instance, previously water would not show behind transparent objects and water rendering in general was very poor. That's now all been fixed meaning that any effects involving water are now much more realistic. Probably the most useful enhancement however has been the added ability to pan by holding down both mouse buttons (left and right). This means you can pan anytime, anywhere: in the middle of drawing a polygon you can pan away and then continue drawing, and you can start panning without access to the background. The developers have also added Czech, German, Spanish and Portuguese translations of the game for the first time. Note that you can watch Phun in action here.
A great little application that proves simple physical concepts in an fun and interactive way.