Universe Sandbox is a very good gravity simulator. It's pretty well done in all its parts.
Well, i think that the best thing is the galaxy collision. Not every gravity simulator can show it.
However, it can be improved: for example if you make the simulation run you don't obtain a new galalxy, but just a big star cloud with the 2 galaxy centres.
The opposite thing is that lagrange points L1, L2 and sometimes L3 aren't stable (however it shows you how difficult is mantain something in that position for long)...
The last bad thing, the less important in my opinion, are the rings made of particles and not of real matter (they can't create new moons or new planets).Can they be created procedurally?
There is a game (Infinity) under developement that create anything procedurally; even rings
Here the video (If you are looking only for the rings part whatch the last quarter, but i recommend you to watch it all):
http://www.infinity-universe.com/Infinity/index.php?option=com_content&task=view&id=45&Itemid=93Here the main page (see also the forums and expecially developement journals):
http://www.infinity-universe.com/Infinity/index.phpP.s. All the video are in real time with an accessible computer and not rendered