Indigo is a free, actively developed, physically-based unbiased render engine.

Employing advanced features such as Metropolis Light Transport, Spectral light calculus, a virtual camera model, physical sky and more, Indigo can achieve much more realistic results than traditional GI raytracers.

Indigo does not need any render parameters like GI samples, AO samples, AA samples, Soft shadows, etc, and complicated lightning setups to achieve realistic results. Indigo is free for non- and commercial use, but as of writing is not open-source.