From Glare Technologies:
Indigo Renderer (64-Bit) 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 depth of field, spectral effects, refraction, reflections, and caustics. Indigo features Monte-Carlo path tracing, experimental support for 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) and it supports subsurface scattering and has its own image format (.igs).