POV-Ray is the main one. POV-Ray - The Persistence of Vision Raytracer I've used it before and it works really well. There are even multi-spectral extensions to account for simple refraction. It has fallen out of the limelight because there's never been a simple easy-to-use interactive modeler. Blender has taken its place.
I keep hoping that PTC will build POV-Ray into their software, or at least create a POV-Ray output. There used to be SLP2POV utility, but the main path I see now uses Proe->STEP->Rhino->POV-Ray..