smallvpt is a small volumetric path tracer written in 150 lines of code, it is an extension of smallpt, which is a tiny path tracer written by Kevin Beason.
It currently includes basic volumetric light transport supporting multiple scattering and homogeneous media. The medium is defined as a sphere surrounding the scene, with parameters sigma_s and sigma_a to control the scattering and absorption of light.
Feel free to play with it as you like, if you render something cool or spot an error, please let me know ;)
I apologize in advance for the noise; my computer is not fast enough to clear it, and I can't afford an overnight render due to the untolerable sound of a choking CPU :)
Without further ado, here are a couple of renders showing what it is capable of: