This Java applet simulates motion in the gravitational field of a massive object. If the checkbox Rel. Corr? is left unchecked Newton's law of gravity is used to compute the motion. If you check off this box, Einstein's theory of general relativity is used. You can choose the initial angular momentum L using the scrollbar. Start and stop the integration with the Integrate and Stop buttons. A fourth order Runge-Kutta scheme is used together with the initial conditions r=1,v_r=0 at t=0. The horizontal line gives the distance scale which depends sensitively on L.
Here's the source: