The original Mitsuba, also by Wenzel Jakob, is an extremely fast, clean, and modular piece of software engineering, in a space with not very many excellent free

The original Mitsuba, also by Wenzel Jakob, is an extremely fast, clean, and modular piece of software engineering, in a space with not very many excellent free implementations; this looks to be quite an extraordinary successor. With the ability to be compiled to run on GPUs, it could even conceivably compete with Cycles, if the community picked it up and ran with it.