THE standard for renderers of quality.  It is either RenderMan compliant, or it is totally proprietary.  

''Exqueeze me? RM is Pixar's PROPRIETARY renderer. And I've heard it's scanline-based, not a ray-tracer. Someone has a hero-worship thing going here. Props to Pixar and their excellent work, but they are hardly the fount of all wisdom. --PhlIp''

A free (in the sense of no cost, not in the FreeSoftware sense) RenderMan compliant renderer is BmRt (http://www.bmrt.org/).

''Well, what's left of the BMRT project looks like it's no longer there, and there's a hint of a lawsuit with Pixar...''

What about Aqsis? http://aqsis.org

Some people think PovRay does a pretty good job at raytracing.