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This reminds me of this old (like, System 5) Mac physics/astronomy program my dad used to have, where you could create little planets and assign them mass and momentum, and give them moons.
The trick was to create little systems that would orbit forever. Man, that was a great program. I should call my dad and ask him what it was called.
iconomy (and others) have you checked out Kongregate? Registration is required, but it's by far the best and least annoying free game site. Just be sure to turn the chat off while playing....it can be very distracting to have flame-wars between fifteen year olds scrolling by while you try to beat Fancy Pants Adventures for the umpteenth time.