Cosmos is an OpenCL accelerated n-body simulator. It can be used to render n-body simulations at a high speed.
Using Cosmos is a two step process. First, you must output binary data of the results of a n-body simulation. Second, you render this to a high-resolution image sequence. Cosmos provides two tools for these purposes, cosmos_simulate and cosmos_render.
Unfortunately, the Cosmos toolchain does not provide an easy way to tweak initial conditions or parameters. However, it does come with a great set of pre-defined conditions and parameters. If you want to modify these, you will have to edit the source code before compiling. If you want to find places where you can edit parameters, search for the text
PARAM in all source files.