Tungsten Renderer!

LuxRender is a physically based and unbiased rendering engine. Based on state of the art algorithms, LuxRender simulates the flow of light according to physical equations, thus producing realistic images of photographic quality.

appleseed is a modern, open source, physically-based rendering engine designed to produce photorealistic images, animations and visual effects. It provides individuals and small studios with an efficient, reliable suite of tools built on robust foundations and open technologies.

NOX Renderer is a rendering software based on unbiased methods.

YafaRay is an open source raytracing render engine. As an open source project, users and developers have free access to the source code and it is free, as in free speech.