Don`t know what is Indigo? Here is a description of Indigo, by Nicholas Chapman developer of Indigo:
Indigo Renderer is a free photorealistic ray tracer. Indigo uses advanced
techniques such as unbiased rendering and Metropolis Light Transfer to achieve
highly realistic results, while remaining simple to use.
Here are the highlights of this new release:
- New Camera tone mapping type
- IES light support
- Network rendering improvements
- Realistic camera aperture diffraction glare
- Region rendering, foreground alpha rendering
- Better filtering for higher image quality
- Improved materials
- Fully functional 32 and 64 bit Linux builds
And yes! It has a full functional build for linux! If you want to download Indigo, visit this page. And, if you want to check out the updated documentation, about the use of Indigo as an external renderer for Blender, visit this page (updated). This documentation is related to Blender 2.45!