Indigo Renderer

Indigo Renderer is a 3D rendering software that uses unbiased rendering technologies to create photo-realistic images. In doing so, Indigo uses equations that simulate the behaviour of light, with no approximations or guesses taken. By accurately simulating all the interactions of light, Indigo is capable of producing effects such as:
Indigo uses methods such as Metropolis light transport (MLT), spectral light calculus, and virtual camera model. Scene data is stored in XML or IGS format.
Indigo features Monte-Carlo path tracing, bidirectional path tracing and MLT on top of bidirectional path tracing, distributed render capabilities, and progressive rendering (image gradually becomes less noisy as rendering progresses). Indigo also supports subsurface scattering and has its own image format (.igi). 
Indigo was originally released as freeware until the 2.0 release, when it became a commercial product. The Indigo 3 series introduced many important features such as realtime editing capabilities, while version 4 of the software adds pure GPU rendering through a vendor neutral OpenCL path tracing engine. 