This is a ray tracer written in C# that I wrote ages ago. It supports:

• Global illumination it simulates the way that light bounces off of objects and illuminates others
• Progressive refinement it quickly generates a full grainy image and slowly adds more samples, converging on the solution over time
• Render pause/resume the render can be paused and resumed later - all data is stored on disk
• Area lights actually, any object can emit light
• Primitives triangles and spheres
• Reflection and refraction and glossiness for both

Cornell box

Shortly after beginning a render

Hours later