here's something that I've always wanted to be able to do: solar system building... do you know about lagrangian points? if you don't, and you aren't really interested, then I'd suggest you to skip to the next post. But if you are, and you happened to read Larry Niven's Puppeteers saga, aka "The known space", youll love this website...
let's see if I can embed one of the java applets... YES!!! click on it, and start the simulation ;-)
impressive, isn't it? the two planets at the lagrangian points are in stable positions, but since their mass is a little too much compared to the central one, they become unstable with time... and one of them ends up thrown away from the system (the pink one)
now be good boys, and visit the page linked there's much more fun to be had!!!
(and this includes a kemplerer rosetta of 48 planets!!!)