As a side-project, I’m working on writing a ray-tracer, with much help (and source code) from Scratch-a-Pixel. This is very basic at the moment – still need to add lighting, shadows, reflections and triangle intersection.
Although ray-tracing is not used in real-time rendering (and probably never will, due to the algorithmic complexity of ray-intersection tests, and not until we get a revolution in hardware at any rate), being able to cast rays is often very useful in other areas, such as AI. Also, it’s a good way of learning the fundamentals of sampling and anti-aliasing since everything is done in software.
Coding Is Magic 