Kerkythea Free Rendering

Based on Physical properties for materials and light, the Kerkythea rendering engine seems a promising development, in the small market sector where Maxwell is getting so much attention.

Give it a try. Linux and Windows version are available and there is already integration with 3ds max, Blender and SketchUp.

Edited: see comment... the renderer is not Open Source, but currently free
I have tried the Linux version, but the download did not work on Ubuntu 64-bit -- maybe on 32-bit?