I'd like to take this opportunity to advertise Visionaray, which is (yet another) ray tracing framework.
https://github.com/szellmann/visionaray
https://github.com/szellmann/visionaray/wiki
Visionaray is based on generic programming with C++. In contrast to other frameworks, its main goal is to achieve platform independence: write "kernels" in C++, have "schedulers" (CUDA, TBB, C++-Threads) execute kernels on the hardware you desire. Users write their own kernels, though the framework provides a few custom ones (primary rays only, whitted, path tracing, ...).
Visionaray is the preliminary result of a hobby project of mine that I mainly pursue in my spare time, and for a few weeks now partially during my work at the University of Cologne. Visionaray is open source (MIT license). At its current state, I consider it not yet mature enough to be any useful in a real project, but we (at this time it's basically a student of mine and me) are working on it.
Companion projects are concerned with ray tracing in VR (a plugin for the VR renderer OpenCOVER (https://github.com/hlrs-vis/covise) comes with Visionaray) and ray tracing on FPGAs (we're working on a Xilinx Vivado HLS scheduler and porting code to fixed point

Best,
Stefan