On his website Kevin Beason presents a Monte Carlo ray tracer written with 99 lines of C++, generating a picture of a Cornell box with global illumination. Beyond the interesting experiment and the fact it can generate a binary of 4kB, I find very valuable the fact there are slides explaining all the code.
Last year Cyril Crassin presented a voxel based approach for interactively computing indirect diffuse and specular lighting, along with a couple of demonstration videos, and kept working on the matter since then.
In this talk given in May at the NVIDIA GPU Technology Conference, he briefly explains the technique:
Interestingly enough, as he points out, the technique has been implemented in the Unreal Engine 4 already.