Principia is a JavaScript library who offers virtual experiments to test and learn the laws of physics. This is not a perfect simulator, but it's good enough to have fun and enjoy the science for a while playing with the Universe in God mode.
Please go to Principia Homepage to enjoy some examples. More examples contribution is welcome.
I tried hard to use internally the actual numbers of the planets, but they are so huge they caused some overflow problems in JavaScript (you know, it is not the best language to have precise calculations because its float number precission limitations), so I had to use an arbitrary number (1e6) stored in the constant K to downscale it all.