This gravity simulation is a sandbox on a grand scale. Put planets, stars, and asteroids in place, give them an initial push, and see where they go. Observe the conic sections that emerge from the simple rules of gravitational attraction.
But gravity has more secrets in mind than even Newton knew. With a single star, the motions are easily predictable, yes. But add another star or planet into the equation, and things become chaotic. Remarkable patterns can appear in a seemingly random way.
Why stop there? Go ahead and simulate what we might observe if gravity deviates from a 1/r^2 law. Change the law just a little, and try to recreate the strange and famous behavior of Mercury.
Throughout it all, though, there will be beautiful and captivating pictures. So go make some worlds, and make some art.
The latest version of Gravity Simulator runs in WebGL, so you don't need to download any plugins to play. If you want to relive the past, you can go back and use the old Unity Webplayer version. Or the even older Flash version.
April 18, 2016: Now built with WebGL! (No plugins needed to play)
May 19, 2014: Dust added, more orbit line options added
April 22, 2014: Updated with new menus and options
March 2014: Released!
If you like it and want even more speed and features, go check out the downloadable version of the sim. Easy to get on a bunch of platforms.