Not really the type of simulator I was expecting, but still kinda cool.
I remember a simulator that allowed you to add planets and see how the interacted with each other. You were able to change the size and density also
how fast they move, It was pretty cool but I have lost the link unfortunately.
Thanks for the link OP. I think.
Needed to click the link a few times and for some reason am unable to see a simulation.
The program Topsy is referring to is called Universe Sand Box
Actually it's quite an entertaining and visually pleasing program to play with. The screen shots on the web page don't really do it justice though.
You can fast-foreward the solar system up to a point. If you run it too fast it tends to eject the inner planets. Presumably, that's because the
planets are only "seen" for a small part of their orbit. I think the program is a numerical simulation, rather than based on general relativity.
Nevertheless, you can track asteroids, like Apophis with reasonable accuracy if you have plenty of computing power and patience.