In my HPG 2009 report, I mentioned that Sony Pictures Imageworks was releasing several of their projects as open source, most notably a shading language, OSL, tailored to ray-tracing. For a long time, there was no actual information available on OSL, but now (tipped off by a recent ompf post) I see that some has appeared.

OSL is hosted on Google Code, the main page is here, and an introductory document can be found here. The language has several features that seem well-designed for ray-tracing; someone with more knowledge in this area will have to weigh in on its usefulness.