It's http://www.orbitsimulator.com actually

Everything in our solar system is still slowly evolving as gravity influences moons and other planets - just not drastically. For rapid migration you need gas drag, which only happens while planets are forming since the disk is full of gas and dust. I'm sure the planets' orbits are changing very slightly because of their influence on eachother, but they're not evolving so fast that we can really spot a huge difference over a few centuries - we're talking geological timescales here.