Create a sun, spin some asteroids around it, and watch them collide and form planets.
An N-Point Gravity Simulator is a realistic simulation in which all points attract each other: even the smallest asteroid has its own gravitational field that can affect even a large sun far away from it.
Can be reused as a library for making games.
If used as library, license is GNU LGPLv3, otherwise it's GNU GPLv3.