Sunflow is an Open Source Global Illumination rendering system written in Java. A new version was released today and it works with Blender thanks to a Python export script that has been made available.
The images in their gallery are pretty impressive although I don't know if they were made with Blender or not. I also wonder a bit about how fast it is, being written in Java.. any comments on that?
About GI renderers (from Wikipedia):
Global illumination algorithms used in 3D computer graphics are those which, when determining the light falling on a surface, take into account not only the light which has taken a path directly from a light source (direct illumination), but also light which has undergone reflection from other surfaces in the world (indirect illumination).
Images rendered using global illumination algorithms are more photorealistic than images rendered using local illumination algorithms. However, they are also much slower and more computationally expensive.