"LuxRender is a new, free software rendering system for physically correct, unbiased image synthesis.
It's an authorized fork of the PBRT project, focusing on production rendering and artistic efficiency instead of academic/teaching."
This renderer is similar to Maxwell Render, Fryrender, Indigo Renderer, and others in that it's based on unbiased rendering. It's dissimilar in that it's completely free and GPL code!"
Link to Original Source