Gravitorium Pro
The prime purpose of Gravitorium is to let you play with ideas about gravity and gravitational systems. While it's not intended to be a highly accurate solar system simulator, at low speeds its accuracy is really pretty good. And, best of all, it lets you "play universe maker". Throw in a few extra planets, add a black hole, set a comet plunging towards the Earth! Or create an entirely new solar system from scratch.