CAD, BIM & 3D

The free Indigo renderer at http://www2.indigorenderer.com is an "unbiased render engine", meaning that it works purely based on physical properties. Similar in concept to the more famous and commercial Maxwell renderer. Free but not open-source (at the moment). Commercial use is allowed with some minor limitations. There is support for Blender, 3ds max/VIZ, Cinema4D, SketchUp, Maya and XSI. Windows only.

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?

On http://www.urob.de/index.php?para=107 you can download a free windows-version of this Finnish modeling, rendering and animation application. This is an older version of the current 5.x version, which is a commercial product trying to compete with 3ds max, Cinema4D and others. The site is in German, but the links are clearly marked. The software is the English version.