I ported a ray tracer I wrote for a class project to java, because I wanted to compare its performance with native code. (It turns out that it's not much slower at all, at least compared to the SGI binary, when it is running under appletviewer).
The code is pretty rudimentary right now. The one thing it really needs is allowing users to enter the coordinates of the objects interactively. I also haven't ported the polygon code so it only traces spheres right now. It would also be nice to be able to specify some colors. In any case, feel free to hack the code up. If you are brave enough to port the polygon code, feel free to mail me for the source. (I probably shouldn't publish the code here since it may still be used as a class project here.) The original ray tracer supports colors and an input list of objects as well.