This application will give you a lesson in how difficult it is to control the universe. Select how many planets you want and adjust their orbits and other parameters. Then set it in motion and watch your planets crash into each other or fly off into deep space. At least that's what happened to mine! With some practice, you might get a real system going. Link -via J-Walk Blog
http://imgur.com/HPgsL.png
@Justin : That is a better image than what I got.
Is it lunchtime?