The second implementation was mostly written in summer 1998. It uses C with a custom object system, OpenGL for rendering and Tk for the user interface. It includes many of the features that were originally described in the proposal. It can simulate a bouncing ball with a fixed timestep, using rules for motion, bouncing, and gravity.
Current status (summmer 2002): I'm just putting some old documents up on the web. It would be nice to generate some programmatic flash movies, but with the fixed-timestep simulation it's not enough fun...