The main web page for the Tachyon ray tracer, written by John E. Stone, is at [http://jedi.ks.uiuc.edu/~johns/raytracer/]. On that web site you can find a short manual [http://jedi.ks.uiuc.edu/~johns/raytracer/papers/tachyon.pdf in pdf] and an examples [http://jedi.ks.uiuc.edu/~johns/raytracer/raygallery/stills.html gallery].