The Ray Tracing in One Weekend series of books
Java distributed tracing implementation compatible with Zipkin backend
CIRT - CUDA Interactive Ray Tracer
Physically accurate and validated GPU ray-tracer
Differentiable RayTracing in Julia
Open-source CUDA/OpenCL speed of light ray-tracer
IMSE use of FusionOptics ray tracer
Fully 3D polarisation sensitive ray tracer.
Gphoton in a simple 3d modeler and ray tracer.
A complete 3D stochastic channel model for urban and rural environment
A really simple (following K.I.S.S statement) ray tracing project.
Exttended 3D 3GPP ITU channel model
Bruno's RAY Tracing Engine