'Nbody' is a small, simple application that simulates n-body gravitational motion of discrete masses using 'classic' 4th-order Runge-Kutta integration. You can create simulations of up to 100 masses, or run and edit one of the built-in examples. The display of motion can be rotated and zoomed in 3-d while running.
All simulation data are saved when you quit the program, and are automatically reloaded next time to start from the same place. You can save edited simulations to 'disk' or the pasteboard for export via email. Simulations can be downloaded from a website or via email as text. There is a built-in user's manual, and a companion website with additional information.