Acronym for Blue Moon Rendering Tools.
A set of programs that conform to the RenderMan interface standard, written by Larry Gritz. BMRT supports raytracing, radiosity, area lights, texture mapping, environment mapping, programmable shading in the RenderMan shading language, motion blur and lots of other stuff.
It runs on
BMRT is not open source, but the programs are free for academic and non-commercial use.
BMRT produces excellent images, although you might need to learn how to program shaders to take full advantage of the programs. It can be used as an external renderer for many other 3D tools.
Information from the BMRT home page, http://www.bmrt.org