Tungsten is a physically based renderer I originally wrote in early 2014 for the Image Synthesis Rendering Competition at ETH, where it won 1st place with the image pictured above. It is intended for offline rendering in graphics research and is still in development, at least whenever I find the time. It lives on Github.
Tungsten is written in C++11 and makes use of Intel's Embree ray tracing library for fast ray-primitive intersection tests. It was written with performance in mind and exploits multithreading and SIMD where possible. On top of low-level optimization, sophisticated integrators, multiple importance sampling and solid-angle-adaptive light selection reduce the overall runtime by battling noise, and adaptive sampling makes sure that effort is concentrated in the noisiest regions in the image. Nobody likes Phong shaded spheres anymore, which is why Tungsten supports many geometric shapes and material models to make scenes more interesting. Participating media are also supported, including an atmospheric medium that can render planets from space.... More Info »
At its current state, Tungsten is more of academic interest than for actual use. Crucial features such as motion blur are missing, and the lack of documentation is making it difficult for anyone but myself to use the renderer. These are things that are being worked on, but progress tends to be slow due to a consistent lack of free time.