Tachyon is a raytracer developed by John E. Stone. Tachyon supports
the typical ray tracer features, most of the common geometric
primitives, shading and texturing modes, etc. It also supports less
common features such as HDR image output, ambient occlusion lighting,
and support for various triangle mesh and volumetric texture formats
beneficial for molecular visualization (e.g. rendering VMD scenes).

WWW: http://jedi.ks.uiuc.edu/~johns/raytracer/