Use the sliders and text entry fields to build a solar system. You can adjust the mass, both components of the planet (or moon's) velocity, and its distance from the center point. Press "Run" to see it progress in time. Use the preset drop down to select some interesting examples.
Solar Systems take a lot of work to make. Most don't work out in the end. Play around and try to make one that lasts for, say a few billion years.
Link to stand-alone sim: Here is a link to view the sim on its own, with no distractions.