Could you, please, add ray tracing tutorials? It's a very wide subject. For instance, recently I was scratching my head about it and still don't have any significant results.
Ray tracing would be a really fun thing to put on here. If you're doing XNA, you don't need a full-fledged ray tracer, per se. But the concepts involved actually go a very long way when you try to make shaders and even in some physics/collision detection setups. So the principles are definitely valid.
But also, if I'm going to expand beyond just pure XNA (which I'd like to do at some point) ray tracing would be a very fun direction to go.
Thanks for the suggestion! (Sorry this response is so late!)