Not exactly a game, but not really an app either.
Create and model solar systems using Newtonian Physics.
See what happens if you put a black hole in our solar system,
or what a quadrary star system looks like.
Navigate through the menus to allow for various options, like adding planets and the like. Left click and drag to move the screen around. Right click and drag to zoom in or zoom out. For any of the menu items that follow or zoom in on certain planets, type the exact name of the planet you wish to track. To cancel the zoom, enter nothing.