Gravit is a powerful, open source, cross-platform gravity simulator.
Gravit is cross platform and it successfully runs under Linux, Windows and Mac OS X. It's released under the GNU General Public License which makes the source code freely available. It uses Newtonian physics to calculate simulations. Although the main goal of Gravit is to be as accurate as possible, it also creates beautiful looking gravity patterns.
Gravit stores the positional history of each particle in memory so it can animate and display a path of its travels. This allows drawing of each particle's full history as a line.
You can also save the whole history of any simulation onto disk and load it at a later stage.
Gravit is written in C and leverages Lua, SDL and OpenGL. It used to use the Barnes-Hut n-body algorithm, but due to complexity and taking advantage of multiple processors, it is currently using particle to particle calculations (i.e. O(n^2)).
Gravit was initially written in 2003 where there were no decent looking gravity simulators around. There also were not many cross platform, open source gravity simulators.
Gravit was called Graviton, but the name was changed because someone already used Graviton for their project.