This little sim could easily be mistaken for a simple billiards table kind of thing. Ah, that is until you find the gravity slider. Adding gravity doesn't just suck all the balls to the bottom - it gives them their own gravity. Which of course turns a simple billiards simulation into a solar system simulation.
Your easy job is to try and get a moon into stable orbit around a planet.
Your nearly impossible job is to try and get a moon into orbit around a planet which in turn orbits a sun.
UPDATE (3/13/06): Link changed. (Thanks Brad.)